“I really need your help. I don’t want to be like this. I want to be a good person.”
Right, well, Tate has just gained a ridiculous amount of sympathy from me. I have been there. I mean, I haven’t nearly chewed anyone’s face off to make a point or defend my crush, but I have been there. (Yes, more American Horror Story, although no spoilers this time.)
Knowing that there’s something wrong, but that even then the mind you use to understand that truth is off. And knowing isn’t enough to fix it, thinking about it isn’t enough to fix it, wanting it isn’t enough, trying isn’t enough…
It’s hell, and there is no-one I’d wish that on.
Anyway. Between Tate’s apparently genuine affection for Violet and the pain he’s in, I think there’s enough sympathy to offset his monstrous behaviour. It helps that the people he hurts are presented very unsympathetically (although Leah seems to be mellowing), and are really minor characters; if they were developed, there’d be more of a chance to build empathy for them and resent Tate for what he’s being. Meanwhile, he’s a monster who’s perfectly willing to put his monster-hood at the service of those he cares about, and he’s trying (albeit with restrictions imposed by his nature–both supernatural and psychological) to be a passably decent guy.
This is an archetype. And I will be damned if I can nail it down. I keep associating the phrase “pet monster” with it, but that seems distinctly off; if nothing else, it puts the character in a subservient role, rather than a devoted one.
It particularly interests me because the same archetype shows up in the story fragments I’m writing with a friend, and we’ve tried so hammer out why he’s staying a likeable character. He’s horrible. He does horrible things. And yet between his misery at what he can’t do and the distancing and compartmentalizing of his victims, it doesn’t quite seem possible to hate him.
 He’s not a heroic sociopath; he’s not a sociopath, and he’s not played for laughs. He’s a stalker with a crush, but that’s incidental to his mental illness. Psychopathic manchild doesn’t fit, because he doesn’t have childlike qualities. He’s not set up as enough of a monster to rate a cry for the devil. And all those links are to TV Tropes, so I advise not clicking if you had free time you wanted to devote to someone else, because that place is a timesink.
 Coroner/killer relationship. Oh, the issues. The deep and serious issues.