Professor Wilkins first seen as he's greets Dexter and Deborah Morgan, while they're searching for an another college professor, Jerry Halpern. He leads the to Halpern, who quickly becomes main suspect but later turns out as a suspect himself.
He easily escapes police surveillance at least once and it is possible that he used the gap to dispose of the body of Kurt Wagner (who was killed either by him, or another cult member, Darius Starzak).
During novel's climax, when Dexter infiltates Cult of Moloch hideout, he shoots Wilkins and Starzak dead with a gun he had stolen from one of the guards.
While he's never seen killing anyone directly in the novel, he was an accomplice to at least 8 murders and also, quite possible, directly killed Kurt Wagner (though it may have been done by Darius Starzak).