As for Stacey...she's young and desperate, he's rich (well, compared to her), he buys her things, he treats her like crap... haven't we all made stupid mistakes like that before?? She just screams low self esteem.
And May, well, my skin crawls when I see that woman. That actress plays that role VERY well, because she scares the hell out of me! I think if I met her in person I would have a hard time separating the fact that she is not her lunatic character.
What about Phil and Stella? He's such an idiot. If he spent more than 5 minutes at home with his son he might realize his girlfriend is threatening him.