And why didn't you immediately beat him with the nearest blunt object?

Go to Lowe’s or Home Depot. Get a doorknob assembly with a locking key. It takes little skill and not much money and only a screwdriver to replace a non-locking doorknob with a locking one - Don’t ask permission, just do it. Then use it. The common bedroom door knob is intentionally designed to be easily opened from the outside with a small screwdriver or the little “key” that comes with it but this is easily opened when locked. Obviously you roommate does not respect your privacy or possessions. Be glad they didn’t steal anything. And of course change the pass code on you computer and don’t use anything obvious like you name or birthday. When/If you move you can reinstall the original doorknob and take yours with you.

And what was he doing with your underwear?