If they don't Maledict the Rovagug cultists they'll presumably go to Abaddon and get eaten. Carissa is - actually not sure she can order that, not in front of Keltham - even if it actually serves Asmodeus better in this instance to let a soul be devoured, you just - can't - what are you even doing, if you're willing to do that -
She is thankfully saved from having to consider this by Keltham's lots of questions, which are directed at Maillol not her but she's the Keltham expert.
She thinks that alter-Cheliax does not have a principled objection to hurting people until they answer your questions, if those people were trying to hurt you and the answers are important, but Keltham said ages ago early on the Project that he wanted people to not do that so they have told everyone associated with the Project to, if it comes up, ask first. Also they're probably not going to take the Rovagug cultists alive because Rovagug cultists famously tend suicidal.