Now, when I say that Rockstar "vastly improved" over Mass Effect's branching storyline idea, I obviously mean to say they "did the complete opposite."

In the final narrative decision of GTA IV, the game forces the player to choose which of two secondary characters (Roman or Kate) he or she prefers, then rewards that decision by killing off the selected character.

The choices you're given are presented under the guise of being either solely revenge-driven or money-driven, but a significant factor which guides the player's decision concerns which secondary character supports which option. Kate tells Niko that he shouldn't sacrifice his vengefulness for money (and that she'll leave him if he does), while Roman thinks Niko should leave his life of violence behind and get money for Roman and Mallorie's honeymoon. As the player has no real use for the half-million Pegorino promises Niko (unlike in Vice City, you can't buy property; any money the player earns past the $100,000 mark is pretty much frivolous), the final decision only partially concerns the question of revenge versus money -- equally, it's about choosing between Kate and Roman.  The choice only works, though, if you think it's all about the money and revenge.

In the Mass Effect article, I said that to ask the player which character he cares about most with the explicit intent of then killing them would be (to use my exact words) "a dick move." I still believe that. Yet even though that's the exact situation we're presented with at the end of GTA IV, it's not framed as such; when Kate and Roman are killed, the player blames the characters who killed them, rather than the designers, for the tragedy.

Had the designers phrased the life-or-death decision in the straightforward terms Mass Effect is known for (specifically near the end, where the game puts two of your companions in danger and literally makes you choose who lives and who dies), the character deaths would have immediately aroused anger and distrust. If the game had straightforwardly asked the player, "Who do you want to live? Roman or Kate?," right before immediately killing whomever the player didn't want to die, the player would resent the writers all the way up until the end of the game. Phrasing this essential life or death choice as a matter of money or revenge was an incredibly clever way of working around this problem.

The result of all this, of course, is a much more emotionally satisfying payoff. Though the actual shooting scenes suffer from horrible camerawork and editing (Roman doesn't even get a close-up as he's shot), the fact that the player has to watch as the character they most cared about dies in Niko's arms adds a much more personal slant to the final mission, and ultimate theme of the game. 

The post-tragedy chases and gunfights are, to a degree (more on that in a bit), designed to give the player the maximum emotional impact through simple virtue of choice. Since you unwittingly chose to have Kate or Roman die, it's your fault that they are killed, and therefore your responsibility to avenge them. Since the person who killed your loved one did so because you either betrayed them (Pegorino) or let them live (Dimitri), the player feels a stronger emotional connection to their respective endboss. The actual gameplay in the final two levels is essentially identical (did anyone else have to retry that boat-to-helicopter jump at least four times?), but the emotional charge behind it is specifically geared toward the player.

Same deal with the theme. No matter what, someone close to Niko dies, solidifying the themes "crime leads to the suffering of innocents"/"crime doesn't pay"/"the American Dream is a lie"/"violence begets violence", but the application of those themes are simply suited to the player's sensibilities by the branching plot.

However -- and I'm sure this wasn't intentional -- Rockstar subliminally rewards the player for taking the more "moral" path (in initially attempting to let Dimitri go) with a better dramatic conclusion. The entire game centers around Niko's character arc, as he goes from nonviolence, to violence, to either mercy or vengeance depending on how the player chooses. No matter what, each ending is reasonably satisfying due to their coming about from player choice, but the more moral act of renouncing vengeance is nonetheless rewarded with a generally superior ending.

Opting for revenge at the end of the game is, from a storytelling point of view, not particularly interesting: Niko ends up pretty much exactly where he was at the beginning of the game in terms of his own capacity to commit violence. If he starts off the game willing and wanting to kill people and ends it willing and wanting to kill people, then his time in Liberty City hasn't really meant anything significant to him as a character. The writers do what they can to make this story branch  interesting for the duration of the game (Roman, for the first time ever, teams up with Niko to help him kill someone), but they've got their hands tied; the player has chosen a path in which Niko cannot possibly change as a character.

Additionally, the player who chooses revenge experiences less satisfying character interactions. Both Kate and Pegorino, while reasonably interesting in their own ways, still potentially mean much less to the player than the characters you've known since almost the beginning of the game (Pegorino doesn't show up until almost the end, and you only get to know Kate through taking her out on dates -- she's not intrinsic to the story, and therefore not as easy to care about). If the player chooses to kill Dimitri and make Kate happy, he obviously receives some narrative satisfaction deriving from the fact that Dimitri's death came about by personal choice, but the final fight against Pegorino can't help but feel pretty damn forced. Pegorino was an angry jerk, but did he really have what it took to just drive up and personally shoot at Niko? Maybe, but no matter how believable his reaction to Niko's betrayal is, it's nowhere near as effective as witnessing Dimitri do the same thing if the player chooses the money.

When Dimitri sends an assassin to kill Niko and Roman in the "anti-revenge" path, it makes perfect and immediate sense to the player. Throughout the game, Dimitri repeatedly calls Niko and makes arrogant death threats; he's one of the only characters in the game to actually gain both Niko and the player's trust through serving as a seemingly nonviolent foil to his partner, Mikhail; and he's pretty much set up as the main antagonist of the game once he guiltlessly betrays you. You know him, you hate him, and if you chose not to take revenge on the bastard, you probably did so with a little bit of hand-wringing and vengeful irritation. He is, for all narrative intents and purposes, your nemesis. It makes perfect sense that he would refuse to let the Bellic family go, even though he's already got his money and Niko stayed true to his word. Why? Because he's the bad guy, and has been throughout the rest of the game. To kill him at the very end is therefore an emotionally satisfying enterprise -- more satisfying than killing some Italian you only met a few hours ago, anyway. 

Similarly, the player doesn't feel a moment's hesitation in chasing the assassins back to headquarters and killing Dimitri with extreme prejudice, simply due to how much Roman meant to both Niko and the player. When going back to replay the other ending, I didn't really feel emotionally dedicated to hunting down Pegorino and killing him: I felt like I was doing it more for Kate's sake than my own. When Roman died the first time I beat the game, however, I felt completely dedicated to finding Dimitri and making him pay simply because I liked Roman so much.

Trying to be a good person results in a much more tragic tale where Niko eventually reverts back to his more violent tendencies anyway, but the anti-revenge ending still remains superior to the alternative. Nobody can say Niko and the player didn't try to turn the other cheek -- Niko makes a full character arc, but is literally pulled back into his violent ways because of Roman's death. He's still a vastly different person at the end of the game, but he also finds that no matter how much he may want to change, the choices he made when he first arrived in Liberty City will still haunt him for the rest of his life. The story of a man who tried and failed to find redemption is much more interesting than that of a guy who didn't want to be redeemed in the first place -- the gunfight at the climax of the anti-revenge ending is not filled with jubilation or bloody satisfaction, but mourning for both Roman's death and Niko's inability to escape his history of violence.

Finally, I just wanted to focus on the final lines of both endings. Anti-game pundits probably wouldn't ever think of using the words "subtle" and "Grand Theft Auto IV" in the same sentence, but the differing dialogue at the end of each ending actually shows a great deal of subtlety and intelligence on Rockstar's part.

After killing Dimitri in the anti-revenge ending, Niko expresses some hatred toward Dimitri and mourns Roman's death, but says essentially nothing else before Little Jacob takes him away. No monologue, no statement of values.

In the other ending, a supporting character tells Niko, "You did it," to which Niko responds: "I don't know...what did I do?" -- a line that seems more geared toward the player than anyone else.

A player who chose revenge as soon as it was offered would need this little unsubtle line of dialogue to explain the theme to them, since they obviously didn't make their decisions in such a way that suggested they'd understand this theme implicitly. Conversely, a player who tried to be merciful as much as possible already knows and believes what Niko would have said aloud in the other ending, and would thus be perfectly content to watch Niko kill Dimitri and silently walk offscreen. To prevent Niko from stating aloud the game's theme is a much better storytelling decision than what we get in the revenge ending, but each ending is specifically crafted to the assumed intelligence level of the player who reached that point.

All in all, the ending to GTA IV is a hell of an accomplishment in videogame storytelling. I'd have liked to see some better camerawork when Roman dies, and the final cut scene after Dimitri/Pegorino's death feels awfully abrupt considering the length of the game which preceded it, but Rockstar still managed to improve upon BioWare's flawed, branching story techniques and develop a truly interesting theme wrapped around a tragic, emotionally effective conclusion.