First: Snape. It wasn't Nagini's venom that killed him; it was the fact that the blood vessels in his neck had been punctured. This is a hard fact of life, but if you get a hole in your carotid or jugular, it's curtains for you in about a minute, maybe two. (If you don't want to say "Snape bled to death," you can say "Snape was run over by the plot." Won't bother me any, and it's the way things are.)
(Now, it's not impossible that someone could have come in right after Harry left and performed some sort of wizarding paramedic spell, but that's for the fanfic to decide. Whether fans believe he's dead or not, the fact remains that he's lying bleeding on the floor, looking into Harry's eyes, and that's when he leaves the canon.)
Second: Snape and Lily Evans. Cue the fannish shrieks of "OMG CREEPY." The fact of the matter is this: He made one dumb mistake, said one wrong phrase in the heat of anger, and he lost what he felt he most desired. And he's been beating himself up for it ever since. And ever since his time at Hogwarts, he's been at least partly driven by the chance to redeem himself in Lily's eyes.
People work like this sometimes.
So when Dumbledore comes along, and offers him the chance at the redemption he's been craving since he was a schoolboy, of fucking course he jumps at the chance. And imagine what it must have been like for Snape--every time he saw Harry, he would remember his love for Lily, but also the humiliation he suffered at James' hands, plus the knowledge that the woman he loved threw him over for the person who hurt him most badly of all. And yet, after all of this, he still stuck to his duty to Dumbledore, no matter how badly it hurt him-- for fuck's sake, it cost him his life and he did it anyway. Even though it was for (arguably) the wrong reasons, he did the right thing. And it's actions that count more than intent in these books.
(Somebody really needs to write Snape and Vincent Valentine crossover fic.)
Third: Harry and his use of the Unforgivable Curses. Fans are shrieking that this is evidence of JKR's overly black-and-white moral landscape in these books-- that if you're one of the Good Guys (tm), you can do anything you like, no matter how reprehensible, and get rewarded for it; if you're one of the Bad Guys (tm), no matter how noble your actions or goals, you get punished badly.
By my reckoning, Harry uses the Cruciatus Curse once (maybe twice?) on Bellatrix Lestrange, right after Sirius' death in Order of the Phoenix (and Bellatrix' reaction is to mock Harry, saying "You've got to mean it!"), and he uses the Imperius Curse two or three times in Gringott's, in order to avoid capture. If I'm forgetting anything, please let me know, won't you?
And true, it would have been nice if JKR had spent a bit more time going into Harry's conscience, conflicted about "If I'm using the same curses that Voldemort did, am I really that much better than him?" However, I think it's brought up a lot in the last three books that Harry and Voldemort are mirrors of each other already, and I think Harry using the Unforgivable Curses is a way to show that. I think that, far from making JKR's moral landscape black-and-white, it brings a refreshing moral ambiguity to the books instead. And again, I think it goes back to "actions count more" -- Harry isn't firing off Cruciatus or Avada Kedavara left and right; by my memory, he does it once in the shocked heat of anger, right after Sirius' death, and after that, only does it when there is no other option left.
Fourth, and finally, for now: I hear fannish shrieking of "JKR gave the finger to all slash fans with the epilogue!! She doesn't care about the slash fans!!!!111one"
I hate to break it to you, but J.K. Rowling never cared about the slash fans to begin with. If you want to criticize the heteronormativity of the Harry Potter books, please, be my guest-- but please also frame your arguments in a way that doesn't boil down to "J.K. Rowling didn't pair up the characters I wanted her to!!"