I am the devil, and I have come to do the devil's work.
Usually misattributed to Charles Manson, in regard to the murders at the home of Sharon Tate. Manson was not present at any of the murders known to have been committed by his followers. The actual phrase, though not as said above, was uttered by Charles "Tex" Watson to Wojciech "Voytek" Frykowski.
Actual quote: "I'm the devil, I'm here to do the devil's business. Give me all your money.