Being a PK (as well as a thief/PK hybrid) in UO is some of the most fun I've ever had in a video game. There's nothing quite like the adrenaline rush of being a fugitive and surviving, picking people off and scavenging what you can before the fuzz arrive.

PKing had a deleterious effect on the community -- I'll admit -- but I had fun as both a PK and as a victim trying to escape or exact revenge. UO (pre-Renaissance) remains the multiplayer game with the richest social dynamics I've ever played. When they finally got rid of non-consensual PKing they totally destroyed this richness. Instead of behaving like real people in a community who work together to solve their own problems they had the benefit of inviolable game mechanics to protect them.

So people's reliance on one another disintegrated and they stopped engaging with each other. Without the threat of violence to settle things, people became anti-social and had no problem being very rude to one another. It was tragic!