There was one time I decided a Noble had pissed me off one too many times, so I build a small magma reservoir next to his room, locked him in, and flooded his room with 1/7 magma. I watched his wounds grow steadily worse, likely screaming in pain as he stood on feet burning hotter and more painfully with every second, until his body could no longer allow him to keep his torso away from his death. He died of blood loss, was drowning at the time, to say nothing of how many physical injuries he had.
One time I wanted to see how the local troglodytes would fare against a Forgotten Beast, who was at the bottom of a small pit that had no walkable path down to it. So I set up a simple trap: first, wall off the path to where the trogs are camped. Chain a cat to a post, and build a bridge over the pit. Tear down the wall, wait for the trogs to decide they want to eat some fresh cat, and dump them on the FB. They lost, the FB had skin-melting spores or something. PUDDLES of Troglodyte, if you don't mind.
But the worst thing I've done was... to my computer, along with all the cats.
This was back before the introduction of the caverns and the magma sea, so digging out the entire map was possible.
And I did.
I embarked with 6 novice miners and a herbalist/brewer, and 6 pickaxes, and all the cats. ALL OF THE CATS.
I channeled away the four sides of every layer of the map, and all but one last square of the bottom layer. The entire map area was ready to collapse, after I pull the lever. I told a Dwarf to pull the lever, and in those few seconds before the pillar disappeared and collapsed the world, I saved the game, and exited the program.
Then I opened up the Raw files.
I made cats very, very hot, but not immune to heat.
I gave them the Sever On Break flag, so that they explode into pieces when hurt. Save the changes, open the program back up, and resume.
The destruction... was absolute, and terrible, and awesome, in the most literal sense of the word.
For the last few seconds of their life, the cats burned. Then the whole world went away, and those underground were crushed. Not that it happened instantly, of course, it took about 12 minutes for my poor laptop to figure out where to put ALL OF THAT FUCKING ROCK. But even after the rock settled, even after the dust settled, even after the pieces of various Dwarves had settled, the destruction was not over. The cats on the surface (and there were plenty of those) were still on fire. Some rolled around on the ground, which was suddenly devoid of grass, mewling away their pitiful last breaths, as their limbs burned and broke and splintered. Some had exploded into pieces in midair, and had been strewn all over the place, leaving the newly-bared ground covered in gore and nothing else.
By the end of it, everything had either been burned, exploded, or crushed to death, save for one Dwarf fortunate enough to have been assigned to pull the lever on the surface, who survived- for a short time- with his one unbroken arm and not a lot else.