He dont deserve a quick painless death
Ya'know what, let's put it like this: that entirely depends on what the end result is. The important part is to not drown in the desire to inflict pain yourself, to the point you become as evil as the guy you're trying to justify that he deserves to die painfully.
Beat his face so much, that his face caves in and he dies from bleeding? Okay, brutal, but whatever. You can live a relatively normal life after that.
But if you end up concocting the most painful ways one can die, and inflict them on the "big bad evil guy", you can't argue you're not becoming a fucking monster by the end of it. We should never root for the hero to become as godawful as the king we're hoping would die.
P.S: Now that I think about it, the idea of: "I am allowed to kill and torture criminals because they're evil so it's ok" is the entire point behind the show / book Dexter. And the end conclusion is that having a happy family is nothing but a facade, a shield to his desire to kill people. That he's a monster himself, and he shouldn't let his kids inherit his demons. At no point is it celebrated.