Three things must be true to unlock the Butcher achievement: 1) Gore must not be set to Low, 2) you must play as a class that has a sword in the equipment loadout, and 3) you melee an enemy to death with the sword equipped.
If Gore is not to Low limbs will not detach and the Butcher achievement will be unattainable. Hit Escape and make it set to Ultra:
Menu > Options > Graphics > Graphics > Gore > Ultra
Join a game or create a custom map. When you spawn, hit Escape, select Switch Squad, and select Cossacks, Frontovik, or K.u.K. Trupp. If you do not see these squads, you will have to switch sides, wait and then select one of these squads.
Switch to a class that has swords in their loadout. Spawn. Equip your sword, run up to an enemy and melee them to death. If the achievement does not appear, try again.