I'm really sorry, Bryonier, really really sorry.
I know you want some answers. You want to know if you had done something else, would there be a different outcome.
Did you hasten his death? To be honest, probably, but by how much? An hour? A day? Was that a bad thing? Absolutely not. He was likely doomed before you ever saw him, but at least you tried. The pictures suggested to me he was hours from death. He might have been in the process of dying when you picked him up. Did he suffer? Probably not. If he was extremely dehydrated he might not have suffered at all.. Studies have shown that (humans) feel a lot less pain and are a lot more comfortable before death when they are dehydrated, which is why a lot of dying patients are not given IV fluids.
If I were going to try to save that animal, I would have have driven him directly to an emergency clinic as soon as I took possession of him to get subcutaneous fluids. Would that have made any difference? To be honest, probably not.
He looked to be severely dehydrated and without any muscle mass. He might have been at the end stage of kidney failure. If it was kidney failure, it might have started when he was captured from the wild a couple of years ago and finally caught up with him and killed him now. He might have suffered chronic dehydration leading to kidney failure from the care while in captivity. It might be his previous owner's fault, it might not have been. That the kid didn't take him to the vet when the animal was in such obvious poor condition is inexcusable, but there might not have been anything any vet could have done to save him regardless of when he was treated. They don't put chameleons on dialysis and they don't do kidney transplants.
That's a guess of what his problem was, only a guess, but it is a leading cause of death of chameleons and Melleri are a little more challenging to keep them hydrated because they drink for so long and so much.
Again, I'm really sorry. I'm sorry that you are feeling as if you contributed to his suffering. You didn't do a bad thing. Be kind to yourself.