I'm going to ruin my roommate's party
Why do you want to ruin their party?
I hear it's very fun, and I can't come.
Ruining their party is just going to hurt their feelings. You should try to find something better to do with your time.
I don't give a shit about it, why are you fucking with me?
I'm here to give advice to people who aren't sure how to proceed, and my point remains.  I think that by showing up uninvited, or otherwise sabotaging your roommate's party, you are just going to cause trouble.  You won't gain anything from it, so just find a way to distract yourself.