After extensive studying of all individuals in DF, I have come to the surmise that the most dangerous creatures in the vanilla game, Are dwarven Merchants.
You may be thinking "dafuq?", and I can assure you, It is a reasoable thought, bu not prpoerly expressed.
Why Them? Well,, first off, they are involved in a phenomena known as "loyalty cascade". The sheer destructiveness involved means that his is obvously dangerous. Since it requires effort on the part of the player, this is not enough.
Dwarven merchants bring all sorts of danger associated with merchants. They attract Goblin sieges, Are often the reason for production of "migrant attracting garbage", and Can single-handedly lead to more danger to a fort then any particular siege, as Forts will be forced to design a method to let them in. This oftem involves leaving a opening open, the dangers of which are nigh iuncountable.
But, Finally, they do a damage that is beyond repair, FPS. The very act of items being on the map, as well as pathing and such, will eventually wear down a fort. Moreover, this defeats the argument in opposing Merchants, namely killing them, since the items wil remain. Metal items will remain even if you pour magma on them. Moreover, their deaths for any reason will add a number of none trading-related items.
However, the final affect is far more sinister then is obvious. The number of Items Actually permanantly decreases FPS.