The best Complete Monsters
are those that are pretty much well-aware of it and Genre Savvy
Making them funny is a matter of making them witty. A funny Complete Monster
who's got a knack for inappropriate (but hilarious) Gallows Humor
in gruesome situations he has himself created is a sure bet. If you avoid cheap jokes and stick to actual cynical wit, it's effective.
Giving the Complete Monster
a realistic (not necessarily good) motivation helps make them believable. A character can have motivations and still
be a Complete Monster
: Wanton, gratuitous or pointless cruelty , disregard for the well-being of others, a penchant for remorselessly harming others for selfish motives, y'know the drill.
You exist because we allow it and you will end because we demand it.