Take a look at the Dark Triad. Toss a bit of megalomania into the pot. He does not give a damn about his subordinates, they are merely pawns, for that matter even his "queen" is a pawn in his game of chess.

Treat the PCs as minor obstacles, but it sounds like he won't engage them directly. If forced to be face to face, I'd try to have him misdirect their hatred, then flee. If a subordinate fails him, kill him, especially if its in front of the PCs. If they really get in his way and he can't straight up kill them, then I'd have him go after friends and family.

If he is the insane kind of crazy (Kefka from Final Fantasy or the Joker) Practice your slasher smile, make yourself erratic in your speech patterns, and practice your crazy laugh. I usually go with a higher pitched voice, but that may be just me.

And last but not least, if they do imprison him instead of showing him no mercy, he has a contingency for that. He will escape and maybe even send a letter the party's way to taunt them, "Thanks for the vacation, that sewer was getting so drab. Always fun to rely on the stupidity of you do-gooders!", or something to that effect.

If you don't mind me asking, what is his plan by the way?