GreatWyrmGold, the problem is not that it's hard to kill them, it's that if a Dwarf kills them, it becomes an enemy of the civ. And a Dwarf that kills that Dwarf becomes an enemy of the civ.
Until all are dead.
Luckily the cougar took the fall, so no probs there 
Yeah, I thought that the cougar dying would make him the enemy of the fort (or, more idealy, that the cougar would kill the dwarf, but that failed to happen before I passed on the save, so yeah).
Come to think of it, will dwarves get bad thoughts if a dwarf rots under ground?
Only if they notice the miasma.