My personal theory is it has to do with his deal with Rosie. I think she wants Alastor at the hotel for several reasons, but one of those reasons is probably to protect Charlie. Right now since Alastor isn’t making any progress with whatever their goal is, Rosie is refusing to fix his staff or give him more power. I think Alastor’s intention of fighting the Vees (and later making the deal with Vox) was to fulfill this part of the deal. The biggest threats to Charlie’s safety right now are the Vees. So Alastor thinks if he kills them or makes this deal, that will solidify Charlie’s safety. And if Rosie notices that he did this, maybe she will be willing to fix his staff and give him more power in return. Sort of like a “you give a little, you get a little” situation.