Was Jason's character....

It seemed kind of like that.

To be fair, Hathaway's character was kind of a tease -- which she has a right to be without being mistreated, but you can't blame the guys for being a little confused either. The boyfriend supposedly got his just desserts when he turned jealous, but Hathaway seems like she earned his exasperation. I mean, if she gets drunk and takes random naps when she's supposed to be meeting someone, they have a right to be angry, but she made it like the boyfriend was a misogynist who just loved to grind her down because, well, guys are controlling assholes or something.

Well, yes and no. He has probably been in love with her since forever and thought that he was finally going to get his chance, but the reason he was a total dick later is because he IS a total dick. If the other guy hadn't been in the picture and they had hooked up, there is a very good chance that sooner or later he would have acted dickily towards her for one reason or another.

I didn't see it like that. To duplicate my comments in another thread:- I think we saw the gradual discovery of Oscar's real nature. We only see him at two points in his life. Doing something mean as a child and 'now'. This is a guy who's life has run down hill, he's inherited a failing bar, never moved on, his house is a disaster zone still full of children's toys and the defaced photos of his (escaped?) ex-wife. His companions are a drug adict and a malable younger guy both, as he probably sees it, even bigger losers he can look down on. Then Gloria comes home and at first he is the nice guy almost forcing his help on her but as events move on his jealously and controling nastiness emerge. I think it's a consistant slow-reveal and very well done - in fact the more I think of it the better Jason Sudeikis's performance seems.

The twist seemed to come out of nowhere. If they set it up earlier that he was mean as a child or that he had violent tendencies, it would make sense.

Instead, in the span of a single scene he goes from normal guy to mass-murderer. Some guys get pissy when they get rejected, sure, but killing millions of people was pretty over the top.