Just from the show I took the point to be that Bill was an abuser but of himself first (as most abusers are). Frank knew how to deal with that. It seemed like it wasn't Frank's first rodeo with a self-hating gay man. In the show, Frank manipulates Bill first, albeit without malice. Right at the beginning, Frank manipulates Bill to get help.

I don't think anyone is innocent in the relationship. The point of the relationship ought to have been that the end of the world makes for strange bedfellows. Not that the end of the world is where you find your soul mate. Which btw mirrors the creepiness of same sex relationships IRL. IMO it's always "wrong," but they try to make it work. Thus you have men taking on some female characteristics. Among other compromises. Thus you have more mutual violence where neither partner is clear physically dominant.