That dude with the shrinking gun in Ant-Man. Just fire it at people who annoy me, and then wipe up a little residue and throw them away. No one has a clue.
I'd want to be It, but with a sense of humor. So instead of appearing as their greatest fears, I'd appear as their greatest annoyances. Two forks scraping together. A close talker. Slow driver in the left lane.
Monster: Cloverfield or that species. What better way to let people you're not ****ing around then rolling their precious lady statue's head down the street.
Villain (or is he a monster?): Jason. He can walk and catch ANYONE and has great aim so good survival skills for eating between the stalking and murdering.
How about Ra from Stargate? Upgrade to a new body whenever you like, live like a king, hordes of minions, travel the galaxy. Good times. Just gotta watch out for archeologists and national guardsmen.