Hellhounds are ferocious and untamable, with desires that know no bounds, but once they have their paws on a human they are surprisingly gentle as if they know humans are weaker.   
They would even go as far as to put their desires aside if they find a human in need of helping. 
If a human that they care about is threatened they will fight and the attacker will know the sight of hell itself. If a human attempts to attack a hellhound they will subdue them but not hurt them since a human is weak