If negative Karma is given to someone unintentionally then it's okay but when someone doesn't like the way you create a post or they just wants to destroy the reputation of a particular member can use this against them since granting Karma is not limited apart from time restrictions.
Any systematic incorrect actions with karma will be found and treated as they should: the abusers will get penalties, all karma reduced as a way of destroying the reputation will be restored. So better not trying to abuse the karma system, it won't stay unattended.