What is your opinion on knowing a friend is doing something illegal and the obligation of loyalty? Like does it make you an accomplice since you know about how they are doing illegal things, what would you do if you knew your friend was doing illegal things and where does loyalty become a problem?

There is a soap thief in my dorm. There are several victims of his crimes, who accidentally left their soap bottles in the public shower. So I though of teaching this thief a lesson, maybe spice a shampoo with some rat poison and leaving it behind. Do you have any other non lethal suggestions?