I don't know if by mean you mean outright cruel without reason, or if you simply mean cruel though with good reason.
Example: I sentence all caravans to death by using my trading post as an entrance to funnel all goblins through it. The reason: the trader's accompaniment kills off goblin sieges effectively. Basically I pit goblins vs caravans. The end result is that, usually, a few goblins get through to my traps after leaving elven, human, and dwarven corpses littering the ground. The benefit to me is that I now get free goblin stuff, free human stuff, free elven stuff, and free dwarven stuff. On some occasions, goblins lose before destroying caravans, in which case I sell to these caravans their stuff back.
So I have good reason.
Also mean but with good reason: all my forts are defended, nowadays, by a series of bridges 6+ z-levels above the ground, requiring goblins to weave their way over them to the entrance. An entire siege can be on the bridges when the lever is pulled. A walled off 'landing pad', an upraised area at the bottom so that the goblins can't get on and survivors can't get off, allows my dwarves to pick off survivors through fortifications, and collect their stuff in complete safety. 30 second genocide. I have had occasions where caravans were on the bridge too. Anyone below would have been hit by a plethora of falling anvils, as it was the dwarven caravan.
Arbitrarily mean: I have kept dwarves in two seperate societies, let them all (on each side) become best friends/married, then killed off half of one side of them to see what they would do. I then let the ragers into the other side. The fortress fell, as one might believe, quickly but efficiently. Love Love Love Love makes society fall down ^.^