Alright, yes, May did go a little psychotic towards the end, but she wasn't really that way at first. Eventually though she just snapped. Maybe what she did wasn't "right," but I still felt for her and didn't hate her even after what she did. It's kind of like if it was a best friend or a family member, you just can't really completely hate them for something they do. There's a part of you that still cares. And I really didn't care about the other characters, except for Polly. How could anyone care about the hot blonde bitch Polly was with? She was a nasty person. As for Adam, I think if he was played by someone else, I might have liked the character more. I just find it hard to truly like characters that Sisto plays, since they are usually dicks or psychos, at least from what I've seen. It's become nearly impossible for me to buy him as "nice." If someone that actually is able to pull of the nice guy thing played Adam instead, there's a good chance I would have felt for him more. Josh Hartnett would have been a good choice, although I don't think he would do a movie like this, especially since he's a bigger star now, it had a smaller budget, and he's not a big fan of "horror" movies... Anyway, I also didn't care about the burn-out with the weird hair. We don't get to "know" him, and he's only on screen for a couple of minutes, which usually isn't enough time to care... Sometimes you care about characters in movies, sometimes you don't. The only one of the victims I truly cared about was Polly. I can't help that I didn't care about the others.
In the end, no matter what May did, I still cared about her. If she would have been some straight out psycho that spent the whole movie killing people, I wouldn't have cared. But, that's not the case. Instead we spend an hour or so getting to know her and care about her, and in the end I couldn't and didn't stop caring.