There was a time on World Of Warcraft's PvP servers, when a meeting between a Horde and an Alliance character didn't result in certain death for one or other of the participants. Maybe there'd be a bit of wave, a flirty little dance, perhaps even some help with hunting down mobs. But an early demise wasn't a certainty.
Since Blizzard introduced its 'honour' system it's been a whole different story. Now players have been given an incentive to kill other similarly-levelled players and certain NPCs (in the form of honour and contribution points). But it's also induced a kind of killing frenzy that's left some areas devoid of NPCs and others rich with lag.