No one ever owes anyone friendship, even. For any reason at all. You don't have to be friends with someone you don't want to be friends with.
not for any gain, save
Go back to the start, do not pass go. "Not for any gain" means not for any gain. You can't say "not for any gain, except" and have it make any sense.
Fwiw, they're painting Snape a lot creepier than even I think he was. I really do think he just wanted to be friends with Lily in the beginning, and he wasn't thinking about objectifying and using her as Nice Guys do to the women they inflict themselves upon.