Well, love is blind, after all. Makes it so much easier to tumble down a flight of stairs. Something tells me the proposal fiasco was the culmination of many issues between the two.
Hank seemed kind of controlling to me too. Maybe controlling is the wrong word. I can see someone in Hank’s position — you know, being the strong one, the “rock” — becoming blind to the reasons why he’s needed like that. Maybe he put too much faith in his love being enough support for Chase, or he assumed everything was kosher, thus ignoring the signs that Chase would freak out. Really, I think he was just in love and didn’t see it coming.
Chase, I bet, has abandonment issues on top of depression. Watching someone come back from the brink of death and then die suddenly, that’s absolutely devastating both initially and long term. The worst part of that sort of combination is feeling just fine, acting “normal,” while your spring coils tighter and tighter — until a trigger comes along and you’re sprung. You don’t even know you’re doing it, and the signs are often so well hidden, ain’t no one prepared for it and everyone gets nailed. (Personal experience here.)
We don’t know all that went down but I get the impression most of it boils down to the aftermath of Chase’s mom dying and Hank being so amazingly supportive that a) Hank shot himself in the foot and b) Chase never learned to cope.
EDIT: And c) the meddling sister bulldozing over any perceived inadequacy. From the brush with her personality, it’s her way — period. And since everyone reacts differently, you can bet she did some damage too.