Butcher is a gear perk that gives melee kills a chance to make nearby enemies flee. It turns brutal close kills into crowd control.
Kill someone with a melee attack and other soldiers nearby may panic and run away.
Wear one of the gear pieces that carry it: Clown Head, Danny's Hat, Postapocalyptica, Scalemail Mask. Each piece only takes one body slot, so plan around the other perks you want.
Good for melee builds. It does nothing if you rarely kill up close.
Pairs well with Brawler for faster melee kills.