I've captured goblins with caged traps. I've already stolen all their stuff like explained here, but now I don't know what to do with them.
Maybe my Dwarfs are happy seeing caged prisoners, but I'm not sure.
If I have to kill them, how do I do it?
Gladiatorial Games There are a ton of different options, depending on your sense of fairness and the blood thirstiness of your dwarves, but the basic premise is the same, build a stadium, with a cage connected to a lever in the stadium (or 2+ cages for pitting goblins vs. monsters you've captured). Make sure the stadium can be made inaccessible. Pull the lever(s) and let out the creatures.
There is absolutely nothing wrong with keeping a Goblin and Kobold Zoo. It's possible for dwarves to have a preference for goblins ("Urist likes Goblins for their Terrible Features"), and since Goblins (any non-
tame unit, actually) don't need to be fed, they will (un)happily remain in cages indefinitely.
Just make sure to build a cage via the build menu, and voila! Instant goblin petting zoo.
If you do want to kill them, you can use i to make a pit over a cliff with spikes at the bottom and then use P to assign them to it. Then your dwarves will take them and throw them off the cliff.
This won't work for goblin kidnappers and thieves(which all kobolds are I think) though as they'll slip away from your dwarves and try to fight/escape. You'll have to manually release them, and have your military dwarves kill them/flood the room(with magma!).
I always create a room for caged creatures with hatch-covered holes in the floor leading into 3x3 fighting rooms below. Each hatch is then marked as a pit using 'i' and creatures can be easily dropped into a room with a dwarf warrior-in-training waiting to try out some killing.
By keeping the cages near the pit designations, my dwarves don't have to move the cages far. Note that there's nothing stopping you from making the fighting rooms several Z levels lower than the storage to make sure larger creatures have broken legs before the fight.