He doesn't and Domen of course assured him that Valya was well cared for in his absence. Even if Ismaele doesn't trust him, it didn't do him any good to imagine Domen lying (he already felt guilty enough for leaving Valya 'behind'), plus there were no obvious signs on Valya's body when he got him, not even signs of regular medical negligence like bed sores or anything. If he had known he would have killed Domen though, that's for sure.

And honestly that's extremely fair assumption to make, especially since Domen wanted to humiliate Valya so much and well, in this state Valya perfectly suited his taste, but I think he despised him so much it went right back to not even wanting to touch him, like in an almost 'I'm too good to sully myself like this' way. Which does mean he def did think about it lol
Completely contradicting this, I can imagine him cleaning after Valya's nurse to really take in the sight of how defensless he now is… Go figure, he really is messed up in a very particular way, it probably made him feel holier-than-thou to 'care' for someone he resented that much, or maybe he found more perverse joy in twisting the knife in after the fact…

Weird to think that before Ismaele got Valya out of there, the person who spoke to him the most was him. If Valya was ever to regain consciousness it would be interesting to see how much did get through him, if Domen was truly shouting in the void or if any of this was the torture he was hoping it was. I think not being 100% sure was probably the source of quite a lot of frustration for Domen.