Apart from being a shardblade in a system where such powerful items were not known, it may be not only the power of the Nightblood that terrifies Vasher. There was a mention in Warbreaker that can suggest that breaths needed for creating such weapon can be taken by force (not given willingly “the way it was always supposed to work” ). This, taken to extreme, could cause genocides, only to produce another Nightblood.