Green’s Dictionary of Slang

(US black) the Devil.

1915		L.R. Dingus ‘A Word-List From Virginia’ in DN IV:iii 180: bad-man, n. The devil.
1938		S. MacManus Rocky Road to Dublin n.p.: The question arose whether the Bad Man Below used turf for fuel [BS].
1939		P.G. Brewster ‘Folk “Sayings” From Indiana’ in AS XIV:4 268: The following terms referring to the Devil: ‘the Old Boy,’ ‘old Hairy,’ ‘the Old Scratch,’ ‘old Nick,’ ‘the booger (bogie) man,’ ‘the Bad Man,’ ‘the Black Man,’ and ‘old Ned’.
1964		B. Jackson Get Your Ass in the Water (1974) 54: The devil said, ‘Look, Stackolee, I heard you’s a pretty bad man in that upper land, / but you know you’re down here and met another bad man.’.
1989		H. Rawson Dict. of Invective (1991) 30: The word has a raft of noxious meanings, alone and in such combinations as bad actor, badass, bad blood, bad debts, bad dude, bad egg, bad girl, bad man.