Other forms: abettors
An abettor is someone who helps another person commit a crime. If you drive the getaway car during a bank robbery, you're an abettor.
If you assist someone else in doing something wrong, offering any kind of support or encouragement, you abet that person. From this verb comes the noun abettor, which is a good word for a criminal's sidekick, or the guy who holds his friend's extra spray paint while she paints graffiti on the wall of a school. The root of this word is the Old French abeter, "to bait or harass with dogs," literally "to cause to bite."