Why am I being sent to kill Peasants by questgivers in Dwarf Fortress? Why are the Peasants living in lairs? Why have the Peasants killed livestock and people? Are these outlaws, or rebels, or what?
The Peasant in question is almost certainly a werecreature. I recommend paying attention to the moon cycle, and going after it around new moon. If you try it at full moon it will probably eat you; werecreatures are REALLY tough in their creature forms.
It's possible that it's an ordinary outlaw, but outlaws usually have enough weapon skills to gain a job title other than Peasant. Werecreatures only gain appropriate weapon skills (Biting, Kicking, Punching) in their creature identity, so their cover identity might not have any useful skills.
I think a necromancer, even one who got run out of town but didn't get around to building an evil tower (so he lives in a lair), would have gained a title and/or race modifier, so he wouldn't show up as an ordinary human Peasant. So that's probably not it.
It is, of course, also possible that it's just a bug. If so, it's the first time I've heard of this one though.
In response to edsobo's comment, vampires don't usually live in lairs; when they're discovered they just move to another town and become a citizen somewhere else. Werecreatures usually hide out in the wilderness though.