They like the hunting, though neither of them really know why. Maybe it is saving children who aren't as lucky as they are, as strong. They save those children from a fate worse than death, and maybe that's what appeals to them.
They don't think so though.
They enjoy the hunt, the kill. The make the witch bleed and bleed hard, and then chop off her head and dangle it from the mouth of her cave. And that isn't for the children. That is for themselves - for their own revenge, for the innocence they lost.
They fight for themselves and if they save someone else in the crossfire then that's great, but not exactly what they meant to do.