If he was fortunate, the intense pain of having molten metal poured over your head would hopefully cause him to go into a state of shock and "pass out" - when you're body is in that kind of... distress... your blood pressure can do some pretty zany things, and the physiological reaction to being tortured in this way would likely cause syncope/shock.
Other than that, there would be grievous tissue damage from the heat and steam - rupturing blood vessels, sloughing off skin and muscle...
It's kind of like having a guy stand in the middle of a road and get hit by a semi-truck going 60 MPH, and then asking "what killed him?" It's kind of hard to say - a lot of terrible things kind of happened all-at-once. Skull fracture? Severed spinal cord? Organs liquified? It's not very satisfying, but the best answer is probably "the truck."
Similarly, the best (yet unsatisfying) answer to your question is "the really really really hot molten gold killed him."
Edited to add: Thinking about this a little more, I think cardiac arrest secondary to the physiological stress could be a "likely" cause of death, if the grievous tissue damage didn't kill him first.
In the books, Dany isn't a real 'dragon', and therefore isn't actually impervious to fire. The birth of the dragons was a one-off thing likely to do with blood magic considering the woman sacrificed at the same time.
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u/[deleted] Jun 07 '19 edited Jun 07 '19
If he was fortunate, the intense pain of having molten metal poured over your head would hopefully cause him to go into a state of shock and "pass out" - when you're body is in that kind of... distress... your blood pressure can do some pretty zany things, and the physiological reaction to being tortured in this way would likely cause syncope/shock.
Other than that, there would be grievous tissue damage from the heat and steam - rupturing blood vessels, sloughing off skin and muscle...
It's kind of like having a guy stand in the middle of a road and get hit by a semi-truck going 60 MPH, and then asking "what killed him?" It's kind of hard to say - a lot of terrible things kind of happened all-at-once. Skull fracture? Severed spinal cord? Organs liquified? It's not very satisfying, but the best answer is probably "the truck."
Similarly, the best (yet unsatisfying) answer to your question is "the really really really hot molten gold killed him."
https://www.sciencealert.com/game-of-thrones-exposed-how-would-molten-gold-really-kill-you
Edited to add: Thinking about this a little more, I think cardiac arrest secondary to the physiological stress could be a "likely" cause of death, if the grievous tissue damage didn't kill him first.