Think it's something to do with mc feeling special to him, as in mc feels special. Garron does a lot of stuff that is unhinged purely because he wants mc for himself and no one else can look at or touch him. Maybe imae eventually kinda likes that, someone like garron putting him on some kind of pedestal even though it's twisted, and wrong.
There's also the aspect that garron always knew about the toxin and does it anyway and refuses to take mcs horn to save himself, kind of willing to die for the pleasure of being with mc, again, a gesture no one else has done for mc. Garron knows the horn would cure him but kill/severely injure mc so he doesn't take it or even let on that he knows about the toxin etc. I'm not saying it's a good reason, but I can see some broken people falling for that kind of 'love'. The obsessive, abusive 'love'.
Just when I though he'd change. .
How tf would they end up in love?? I'm genuinely curious