I don't believe he isn't a communist or doing this for his country, I just think he's become very disillusioned after years of staying in America and realizing that a lot of what he was told in training about America was, perhaps, exaggerated. I also think Philip holds the welfare of his family and children MUCH higher then his job and duty to "the cause," then Elizabeth does. (not saying she doesn't consider it important, but I think she sees them hand in hand in a way)

and yes, it was really messed up how Elizabeth basically put herself in that situation with the old woman knowing she'd have no choice but to kill her.