PK'ing got very stupid under certain conditions, anyway. The worst it got was in a relatively hardcore Tibia server (EXP x3, I think, which still was incredibly grindy because Tibia RL itself is grindy as fuck), where there was a single safe area in the whole game, and it was the small room in which you respawned whenever you got killed, with a single exit. There was another safe area in the bank building, but you had a long way until you got there from the spawn. Another mechanic in the game involved not being able to regenerate HP and mana unless you were outside safe zones, so go figure. The game automatically activated PvP mode whenever you got to LVL 20, and there were absolutely no level restrictions.
Sounds fun, but it got stupid real quick since there were absolutely no limits put in place. Yeah, there wasn't even a level cap, so you could practically keep leveling forever. Penalties after death were also pretty hardcore: you lost several levels upon dying, your whole backpack (quest items included, although most quests contemplated this possibility) and usually even some equipment. Dying was a pretty big deal, and even despite the PK "penalties", the potential benefits greatly outmatched them. So players with level 250 tended to wait in front of the spawn point to kill newbies endlessly in single attacks until they dropped everything, or until they got bored, or until they got to level 19. In exchange, the system gave them a PK skull, which gives you slightly larger penalties upon dying (if you ever die at all, with that absurd level) and a small honor incentive if someone decides to go PKK on them. Getting to the deep ends of the honor system awarded you with optional pig and hero skins, for infamous and famous characters, respectively.
Never managed to get past level 30 because there was this single guy who enjoyed spawnkilling low levels. A truly joyful experience.
That's not to say I didn't have fun with that game's PvP system. I still remember trying to identify potential PK and trying to avoid them (although that was impossible in the spawn), or apologizing to another slightly higher level player for accidentally throwing a spell against him. or this guy who PM'd me advising me not to go into the depot's balcony, which was not protected against PvP, because one of the guys waiting there told him he wanted to kill me as soon as I got there; I did it anyway, going full frontal against the guy, and he managed to kill me but not before the last tick of my poison killed him as well. We got back there and started fighting again and again until we lost all XP points and had to stop due to falling below level 20. We later laughed it out, and overall ended being a friendly experience, mostly because it was fair. But getting spawnkilled without being able to fight back or even recover from the last attack? It was frustrating.
I liked how Wakfu handled PKing. Abusers would get a PK flag on them, and upon dying, would get thrown into a jail, where they had to wait some minutes depending on their crimes to get out, without the possibility of logging out and waiting it out (in fact, you could mine coal in jail to get out of there more quickly). There also were player guards, who voluntarily joined their nation's army to protect it against invaders or outlaws; think of a virtual police who got intel on PK to hunt them down. I actually wouldn't mind a game with heavier death penalties and wild PvP if it had some systems put in place to discourage going around killing people mindlessly just because you have a high level and good equipment, and people who are higher level than you just don't give enough of a fuck to stop you because there are no incentives.