Honestly, I doubt that they were in the castle because Moash decided that they needed to be there. I don't think it was his desire for revenge that put them there. I figure that he trained them and then the singers sent them to the palace. Sure his actions resulted in them being there, but I don't blame him for their deaths. maybe they wouldn't have been there had it not been for Moash, but I think that if we're blaming people it's th fused. AT the very least I would say that it wasn't Moash's intent to use these singers as a means of revenge. Although, I have not read Oathbringer in a long time (currently reading WoR) so it's quite possible that I'm misremembering things.
I find this so interesting! I'm so glad that you pointed it out. There are so many similarities between Kaladin and Moash. I find this one really interesting because it's Moash doing things that Kaladin taught him to do. Kaladin trained him and helped him become the person that he is, and now he's so dangerous and deadly because of Kaladin, and he does things like train these singers because even though he switched sides, it's almost like he wants to continue to emulate Kaladin.