I wonder if we'll ever get to find out exactly what Abby signed away when agreeing to that infernal contract (I think that's about the only thing I'm sticking around for, at this point).
Perhaps Abby didn't sign away anything.
The whole point of the contract scene seemed to be more about Ilsa trying to punish Chloe for getting attached than luring Abby to the dark side. Given how happy the transformation made Abby and since we haven't seen any negative consequences towards her (or at least none that seem to have bothered her too much) possibly Slash-stab recognised Abby as a kindred spirit of the succubi and was just facilitating her transfer.
I was gonna clarify but somebody beat me to it: These guys aren’t slaves. They’re men damned to Hell. A key difference.
But, I’m late to the party on this one so, carry on!
"Rather than simply enjoying these comics, we log in to these forums to argue about them. [...] We are some strange people, and I guess we have nothing better to do." - lordoffiling
Sure, they're damned souls, but as we've seen from the beginning, Hell is far from above trickery, coercion, and force to bring people down there regardless of their deeds in life.