One who is responsible for committing a stupid act.
One of my roommates left the oven on. Again. Okay, guys... who's the derpetrator here?
Adjective- The person who is responsible of someone's retarded act and or retarded way of saying something.
"Oh man, I totally just slipped on a banana peel Tom threw in front of me."
"LOL Tom is totally the derpetrator of this situation here."
"I hate Tom with a burning passion."