Is mustard gas worse than napalm?
It depends on who's on the receiving end. Gas is quite unpredictable. It often floats back to harm the delivering side. And the ebb and flow of the battle can bring the delivering side into its effects, as well.
Napalm and its close cousin the flame thrower are far more predictable.
Dead or maimed is dead or maimed, IMHO. The only big difference is how discreet the weapon is.