What makes Borderlands 2 so great is that it is a "gamer's video game." Now, I don't mean this in any sort of elitist "only true gamers can appreciate the magnificence of Borderlands 2" sense. Rather, Borderlands 2, taken as a cultural artifact, only makes sense coming from a background of "gamer" culture (I use the quotes because this culture is not just one of video games, but includes a penumbra of other works like science fiction and action movies, comic books, and literature that are related only in the sense that they are all things that "gamers" like, however you might be inclined to identify such people). This may not sound like much of a strength, but I believe that Borderlands 2 manages to thread the needle of using its cultural context to enrich and inform the game without seeming to exclude or alienate players who don't share the culture (although, admittedly, this comes from someone who is on the "inside").
It's not just the game's numerous pop-culture references. The world of Borderlands 2, Pandora, is a video game world. Beyond simply being "a world in a video game," Pandora operates on video game logic. Its inhabitants act like video game characters. Everyone you meet is ludicrously bloodthirsty. Violence is treated like an amusement. Killing is a hobby. And that's just the "heroes." Outside the walls of Sanctuary, everything (with the exception of certain quest-giving NPCs) is constantly trying to kill you.
And maybe, in an idle moment, you might think that perhaps Pandora is a savage dystopia, and that, far from being a struggle between shades of grey (the notion that it is a black and white conflict is too absurd to even entertain), it is really just black vs slightly smudged black, and in the end the only difference between Handsome Jack (the villain) and Lilith (your ally) is that Lilith is not a sadist - to the degree she takes childish delight in her gruesome murders, she does not specifically find pleasure in the suffering of the people she kills.
And when you think those thoughts, Borderlands 2 can seem like a really grim story in a bright, candy-coated wrapping, but then, the thought creeps in . . . killing things is the fun part of the game. Having a ridiculously high kill-count, complete with exploding heads and human enemies flailing around as they're consumed by fire and acid, is kind of the whole point. So what's really going on is that the characters' agendas align with yours as a player.
Can you really condemn that? Would it not be hypocritical to do so?
That's the brilliance of Borderlands 2's writing. It doesn't matter. Thought about logically, the characterization is monstrous and worldbuildng untenable, but when you're actually playing it, it somehow tracks right along with your thoughts. "I want to go to that place and kill everything I see. Why? Because I love explosions, and they might have valuable loot. Oh, I get to do that? GREAT!" The result is that the game feels comfortable in a way few games can manage. It's rare to have a game that uses its medium to such a profound advantage (off the top of my head, I can think of Brothers, Bioshock, and Saints Row IV.)
Now if you'll excuse me, I have to go murder some people.