I've been experimenting a bit with ways to execute goblins and other beasties caught in the cage traps. Sadly, my first attempt didn't go quite as planned. After pitting all the cages, I noticed several gobbos and other weaker annoyances milling around the bottom of the pit, with varying degrees of wounds. Mostly broken limbs, one or two internal injuries from the spikes, but no fatalities. Quite disappointing. I suppose it's only a 5 or 6 story drop, and I think they cushioned each other's fall. Now, there's a thing about falling on spike traps. There's a chance to dodge the spike as you hit it. If you fail, well, spike. If you succeed... crazy fucking XP. So, this one goblin, out of about 5, dodges as he lands, only breaking his shin. You can see it in the log... he starts as "goblin lasherdwarf" (uses a whip as a weapon), and after breaking the shin is suddenly "furious goblin lasher", furious begin the goblin equivalent of "ridiculous badass" I didn't notice this right away, else I would have found a more magma-intensive way to get rid of him. Instead, I send in the troops to finish them off. I unlock the door and all hell breaks lose. The 4 other goblins go down instantly, already wounded, before my mighty steel-clad and bearded horrors. But the furious lasher... oh my god. He only actually killed two dwarves outright, lashing them in the head and shattering their skulls, but he actually managed to incapacitate all 20 of my military dwarves, and 6 more civilians I hastily drafted to contain him. He limped up and down the main staircase, the very center of the fortress, breaking or fracturing the bones of every last military dwarf I have. ALL OF THEM. But he didn't kill them. He left them like that, curled up in pain, immobile or unconscious, I suppose because that's how they had left him, at the bottom of a pit with a broken shin, for weeks.
I realized there was no way to kill him. Not a single soldier could score a hit on him. Most civilians ran faster than him (what with the broken shin), so he wasn't as dangerous as he could have been. He did manage to break my broker's legs, which is very inconvenient.
How did I get rid of him? Well, I realized that he had nowhere to go, as my gates were closed (this happened right at the tail end of a human seige). When the humans eventually buggered off, I opened the gate, and he made for the exit, to my extreme relief. He almost escaped too, but he got caught in the same cage trap at the front gate that he had been caught in at least a year before. You can be damned sure he's not going back down that hole. I am considering displaying him in my dining hall, for all my dwarves to torment and spit upon, but I'm not going to attempt to move him down there until at least some of my military recovers. Meanwhile, I'm digging a far more deadly pit with a magma backup, just in case I accidentally turn any other prisoners into gods of war.