You can’t be sure he wasn’t set on fire by someone else ! Plus he might have changed his mind, or extinguish on his own later than you’d have out him out, staying alive and suffering even more.

I’d maybe make an exception for someone calmly sitting as they burn, as I think the Buddhist monks who did this in China were. In those cases it’s pretty easy to see they wanted to be on fire.

But a flaming flailing guy needs to be extinguished, whatever the reason he is on fire.