Yeah, I'll echo what everybody else said about this. Very powerful. I read it yesterday, then again this evening. Love is a veiled weapon here; it could have just as easily saved Snape rather than twist him. Coupled with such torment...well, there are plenty of real world examples of how deeply bullying and abuse affects people, specifically teenagers.
I really enjoy any fic that fleshes out a solid characterization of Regulus, and your Snape broke my heart. Another fic that could be canon, yes? ;)