I approach it like this:
If my PC has become so angry to subdue your PC and not out right perma kill them, and you want to act like Master of the Universe and nothing can hurt you, that just increased your chance of death by about twenty percent.
When you're laying face down in a pool of your own blood, it's not the wisest of moves to begin insulting my PC's mother. If they've been willing to take the conflict that far, it's more than likely they're willing to carry it all the way.
After all, let's not forget, almost every PC is an adventurer. They slaughter orcs, duegar, and other sentient beings just about every day of their lives. What difference should it make that this one just happens to have a human face?
My suggestion: If you run into a PC who is unwilling to submit to your will after being subdued is simply to go all the way. But always stay IC no matter what happens.
If your PC for some reason, could never kill another sentient living being (and actually RPed it out on quests and what not) then they probably should not take the life of that PC. Likewise, if you play a badass with no fear of death, who would call my PC's mother a whore while picking up their teeth that my PC just knocked out with their boot, then do that. Just be prepared for the logical conclusion.
However, in the end, no matter what, stay IC. Nobody likes to die, but it makes the whole thing sour for everyone when OOC hate / anger / frustration enters the picture - from the DM's who oversee the PvP on down to the players involved.
Just accept the fact that, playing on EfU, every PC you create, no matter how powerful or how long lived has an ending. Some endings are epic and some endings are not-so-epic. Your PC simply will not live forever, even if they are so sweet to the point of being sick and make no enemies. Someone will eventually try and kill your PC and be successful. Or you will find yourself in a situation with NPCs or the Gameworld that will result in your death.