Obvious question - if your roommate saw you putting it back, that meant he was there in the apt at the time, so why didn't you just ask if you could use some in the first place and avoid all this nonsense?

If you thought you were coming across as the hero of this little story op, you might want to reconsider. This is how I read it -

"I sneakily stole something from my roommate instead of simply asking to borrow it because that's the sort of person I am and I have no idea how to respect social boundaries, and then when he tried to protect his stuff, I went completely overboard in return, because I don't know how to behave around other people. Btw another flatmate was tortured hahaha but that's completely normal right, happens all the time, btw I'm totally normal and not a Craigslist weirdo at all."

I feel desperately sorry for your roommate, poor bastard, my sympathies lie entirely with him.