Honestly, I think both endings are terrible. I'll do my favourite thing, a breakdown! This is just my opinion about it, and we all know about opinions!
The original ending:
It just isn't fluid or believable. They're supposed to be like vampires, ravenous creatures of the night as depicted throughout the movie (although with glimpses of intelligence and a hierarchy). Then all of a sudden, they have an emotional turning point. I know when I'm trying to beat my head through a wall to get to an infamous villian with all my rage and anger, I'd just give up and communicate with hand gestures. It really comes out of nowhere. It's so akwardly contrasting that I ultimately feel bored seeing it. Like I'm being shown the inner secrets of a magic trick I've been trying to figure out on my own for an hour. There's either not enough or too much substance in the rest of the movie for that mundane plot twist. And I know how much I'm just a barrel of smiles after the realization that I've been focused on the annihilation of an evolution of humankind I didn't agree with for the passed few years. Not a single feeling of remorse or guilt at all! With some tweaking and editing, it'd work out. But as-is, I think it's a paperweight. It still doesn't really draw the conclusion that he's a monster. "Oh, I kidnapped a loved one, my bad. Here she is. No harm, no foul." He's been murdering and torturing them for years, there should atleast be some more growling and foul looks. Really show that hate!
The Hollywood ending:
It's Hollywood, 'nuff said! Sure, it's much more Hollywood than the other, but I think it's more plausible. Sure, you completely lose the whole idea of the book within the confines of the movie; but knowing that he was the antagonist the entire time makes the ending even better. He's so stubborn, he ends up taking his own life to rid a small group of them and save his cure. It kind of parallels a lot of events throughout history where groups of people thought they were doing the right thing by doing some of the most heinous acts. But of course this is just an ideal ending to throw it in to the action flick category to churn up a box office hit, I'm just buttering it up a bit.
I haven't actually read the novel, but I'm willing to bet neither ending gave the book justice. With what little it reveals on the Wikipedia, I'd say he stumbles upon the truth himself, not just some sudden turn of events. When he freaks out at his home with Anna and Ethan, I thought that was a good step in the right direction. But it doesn't really mean anything through the rest of the movie. The whole Captain Ahab theme just makes the ending they went with more to my liking. But I guess you're lead to believe he's a legend in the sense of him being a hero instead.
Minus the ending, I thought the movie was excellent. Something about it just strikes a chord.
