I dipped my bullying roommates' toothbrushes in the toilet then placed them back in place for revenge.
Why would you want to dip their toothbrushes in the toilet?
Because they are mean to my friends when they come over.
Getting 'even' with them by tampering with their belongings doesn't fix anything. Why not vocalize the behavior you're witnessing to them instead?
Talking wouldn't make the least bit of impact. I don't care if they are my roommates, they should respect my friends and guests.
I totally understand. Regardless, respect is a 2-way street. You gotta give to get. Politely tell them they need to be nicer when your guests are over.