A perpetrator is someone who has committed a crime — or at least done something pretty bad. You've probably seen cops on TV trying to get a description of the perpetrator of a bank heist.
The word perpetrator is so often associated with crime that police officers and other criminal justice types sometimes abbreviate it “perp.” A perpetrator is often a suspect until it has been proven that he or she carried out the offense. The word usually describes someone who's committed a crime, but any wrongdoing will do. If someone took Uncle Bob's cake and a trail of crumbs leads to your bedroom, your mom will know who the likely perpetrator is.