Experts from a US museum believe it could be one of the oldest examples of advanced surgery. The Museum of Osteology in Oklahoma said the skull is reported to have been that of a man who was injured in battle before undergoing surgery to implant a piece of metal in his head to repair a fracture.
I'm no expert, but my first thought is, molten metal is so hot it would probably do more damage than it would fix. Also, the shape is quite neat/precise, rarely the case when pouring a liquid on a curved surface. There doesn't seem to be any pooling in the cracks and depressions. The edges are very clean.
Furthermore, and to me, most convincingly, it looks like it's been hammered. There are no bubbles, thickness looks regular, the surface looks like it has been worked on, there are even what seem to be folds, to adapt the shape to the wound.
The bones have fused back together, which shows that there was healing, which takes time. So that indicates that the person survived the wound (and the procedure).
So that's what I would guess based on those photos.
I'm very impressed it worked. Look at the wound. Someone seems to have had their skull crushed in by a big blunt object, rock or hammer or whatever. I wouldn't really expect anyone to survive that, even with modern medical abilities.
Edit : I think I sort of misread your question. But either way, the info above still stands.
I would add, re: the execution idea. In most early civilizations, metal is a rare resource. Why waste it one someone you want to kill?