@hkzkthx
Because his skill is rather inconvenient for physical and mental torture and it's a great warning to stop people from trying to play hero.
Considering his goal isn't just to kill his targets but also to interrogate and torture them. His skill is actually not geared towards that goal all. And it's very easily to cause collateral damage, specifically to people that may want to hurt him, but are simply innocent people being used by evil people.
If they weren't already, all the other people he's planning to kill are warned now. He's already put himself at a disadvantage for the sake of drama. Just pick them off one by one while they're alone, and people might not even suspect it's murder and instead think it's a curse or so! His plan, if he has any, is terrible in any case.
@Undorian if the ex-members of the hero party just start to die out of nowhere, I think the others would still pick up there is something wrong, no matter what he does.
@Gagweeb eh, for the most part, this is the same as the novel tho, this part in the novel was just some talk, Sia breaking everyone weapons and armor, then taking Irina to the church
the real part start at the church, with her talking and chrome torturing her, then fooling her and recording her voiced while confessing the party sins/crimes, and then turning her into a monster afterward