I'm going to break something of my roommate's.
Why would you want to break something of theirs?
They are taking the side of another girl in an argument I'm having.
I get being mad about them not taking your side, but breaking something of theirs is not going to help. You will get them even more mad at you, and they may kick you out.
The other girl's stuff is cheap and ugly, a lamp won't cost much.
It's not about the other persons taste or how much it costs. It's theirs, not yours, and you have no right to break it.