Okay, I can't understand why this happened.
My sheriff is also part of a military squad assigned to actively train. He suddenly stops training and runs across my fortress, changes a few levels then goes to my master stone craftsman who is working dutifully in his shop, making masterpiece after masterpiece and just guts him where he stands then just walks off. WTF? I checked him and he's in good spirits, most everyone is except a few of my recruits because of the training. But they are still fairly up there..
Sounds like Justice.
Did you fail a mandate imposed by a noble? Noble mandates being ignored is a crime, and a random individual with that job will be assigned as the gulty party. (I'm not 100% sure if it looks for dwarves with the labor, or how it makes the decision.) Criminals are placed in jails, if you have one, or beaten with weapons if not. Note that if these weapons are effective (like swords, axes and such) then the justice-administering dwarf can cause Fun for your criminal.
Solutions (in varying degree):
1. Magmafy nobles on sight to prevent mandates from ever reaching the level above unconscious thought.
2. Do not appoint justice dwarves - note that crimes are backlogged, and appointing one later can result in multiple consecutive beatings if you forego one and install it later.
3. Give the dwarf responsible for justice an ineffective weapon, like an adamantine hammer, or (in a pinch) a training sword. Do not allow them to equip something else. Make sure they have low skill with this weapon, and prohibit them from using an effective skill, because these weapons can still kill in the hands of skilled dwarves.
4. Alternatively, meet all mandates.