So, in my quest to be crazy prepared and building a massive ice fortress that handles all threats through direct combat, without using any traps at all (except the occasional cage trap to stock the arenas with), I have weaponized dwarven babies.
One of the problems with military recruits is that they are squeamish. Often times they are miserable after their first battle, and may go berserk or throw a tantrum, causing further carnage. In worst case scenario this can lead to a tantrum spiral.
Eventually dwarves will become used to being traumatized, and will no longer care about anything. When they have this trait seeing dwarves, friends, or even lovers die doesn't seem to cause a negative thought. They just don't care anymore.
In order to do this dwarves need to be slowly inoculated to misery. If they're hit with a sudden trauma you could get a tantrum spiral going, but if you get them used to tragedy they will no longer care if dozens of dwarves are slaughtered.
To do this, I have a barracks at the bottom of a long pit. New recruits are sent to that barracks for training. While training, I periodically load a dwarven baby or child onto a retracting bridge above the pit, and drop them down the pit. They hit the floor and explode into a shower of giblets. This of course horrifies the dwarves in the barracks below, making them all unhappy or very unhappy.
Wait a few months until they recover, then drop another baby. Then another one. And another one. Eventually they will be so traumatized by it they just won't care anymore. If you want to do this to your fortress at large instead of a barracks put a meeting zone at the bottom of the pit. This means even civilians will be all used to tragedy, and so you can recruit them directly into the military and they will already be hardened right from the get go.