And how many people has he murdered lately in those silly two-themed crimes?
I'm not keen on capital punishment, myself, even for lesser-monsters like Light or Two-Face (lesser compared with, say, Luthor or the Joker) -- but once you're dead, you're just meat. A sack of rotting organs.
It's creepy, sure, but if the bodies are still a threat post-mortem, then they need to be watched. As for respect? Again, just a pile of organs. Whether you killed a dozen, or thousands, I still don't give a crap what happens to your body afterwards. The level of villainy doesn't matter, you know? You're still a villain. Why exactly *does* your dead body deserve respect, when you exhausted what little respect anyone had for you while you were stealing from and killing people?