I used to play a game called Tibia; they had 3 options. Non-Pvp, Pvp and Pvp-Enforced. NONPVP is obviously self explanatory.
In PVP, players would get a white skull if they attacked a player. If you died you'd lose your backpack and % chance to drop one of your equipment/wpn.
If a skulled playered had an unjustified frag (killing someone that didn't do anything) and got a lot of frags, they'd get RS (Red Skull) and on death they'd lose all their items.
If you continued to frag like that you'd get a black skull, can't attack unmarked players, can't use aoe spells or runes, and take 200% dmg instead of 50% in pvp.
A yellow skull indicates that someone attacked you (while white/red/black skulled), and you won't get an unjustified frag for killing them
An orange skull is a 'revenge' skull, if someone killed you, you're able to attack them without getting a white skull.
party member skulls (green) can kill each other without getting a white skull/ unjustified frag
PVP-Enforced is essentially treating other players like monsters, you kill other people and get exp for it. This became a huge issue when higher lvls started killing new players though
I loved their PVP system though; you can get PK'd anywhere except protection-zones which are only in big cities in 1 or 2 small areas. Sometimes you'd have to run a good 10 - 30 minutes before getting into a safe area.