First, loved and lusted for are two different things.
Second, Gaunter is pretty damn clearly evil, or at least, really bored and using humans for his amusement, he has no qualms about killing random people for interrupting him, cursing them into monsters for refusing him food (i mean, that woman was a jerk but still...), and then there is that whole story with Professor Shakeslock, the rune circle, his dreams about his daughter that he never had who ends up dying, horribly...yeah, it's all pretty fucked up...
And three, sure, these people knew who they're dealing with, and they're retarded for dealing with the devil himself, but i wouldn't say they fully knew what they're into.
I mean, i'm sure that Olgierd did not know just how badly it's all going to spiral out of control and go to hell because of his, well meaning, wishes, and that's the whole problem, as someone above me said, people get what they wish for and not what they actually want, it's as if you told O'Dim that you want to jump really high, and so he would turn you into a frog, in theory it's all good, in practice he's just an arsehole.