While seemingly at first a mild-mannered accountant, he's in actuality a hit man out for Frank.
He already proves how efficient an assassin he is; able to fool everyone with his disguise.
He first appeared in his accountant disguise, arriving at Perfection; most likely trying to track down and capture Frank. Rosalita uses him to get close to Jack and Delores when Jack was doing his Graboid Tour business much to his annoyance.
The Hitman reappears near the end of the episode, "helping" Frank by giving him a ride. When Frank asks if he is going to Las Vegas, the Hitman reveals his true nature and points a gun at the man, telling him that he has "business" in Chicago. It's heavily implied the Hitman killed Frank.
|"Damned jeep trail gets worse every year" ―Earl Bassett[src]|