Mutilate - Inflict a violent and disfiguring injury on.
Oda went 22 years without having the Straw Hats kill a single person, and that's even using lethal attacks against fodder. Then just out of the blue, using a pin knife, and for no explanation at all, Zoro becomes a killer.
And the kicker here, is Zoro is being forced to commit seppuku for being framed for murders he didn't commit. So to clear his name for murder, he.... Kills the person accusing him of committing murders he never did in front of dozens of witness'. It's completely illogical.