Nerf grievous damage from players by like 5%, or nerf the maximum amount of damage from concussion, because the RNG variance on that is retarded. But add in a murder skill that goes:
"Increase grievous damage to players by 5%, but also take 5% more grievous damage."
Basically if a player with the skill fought someone without it, the grievous damage would be the same.
But if 2 players with the skill fought each other, they'd deal 5% more grievous damage to each other. Thus, players that want to PvP can "opt-in" for the murder aspect of it.
Implementation may vary. It could just unlock a hearth magic or toggle that does this effect for 24 hours so buying the skill isn't a noob-trap, and people being unprepared for a gank don't get wrecked.