"terror is as terror does"
Sorry for the facile Forrest Gump paraphrase, but that's really what it boils down to. A terrorist isn't defined by nationality, ethnic group, or religion, but by actions. And one part of judging the actions of terrorists is their choice of weapons--terrorists choose not just weapons that kill lots of people, but that do so horribly and dramatically, to instill the psychological effect of terror.
Like napalm.
Where is it written down that napalm is illegal and if it is illegal are other incendiaries illegal and if not why?
Napalm is a terror weapon, whether or not it's illegal. The illegality of napalm is pragmatically defined by the fact that civilized nations have signed treaties eschewing its use. The United States has refused to give up the use of napalm, making us rather less than civilized.
Napalm is a weapon of mass death because its effects are indiscriminate. It's hardly less so because we have other weapons whose effects are also indiscriminate, like other incendiaries and air-burst heavy munitions and nukes.
Further, the use of WMD like napalm is entirely inconsistent with our avowed concern for avoiding civilian casualties. The less precise the effects of the weapon, the more likely is the collateral damage. That we used napalm in Iraq gives lie to the sanctimonious propaganda about our so-called humanitarian approach to war.
It all sounds a bit ridiculous when people (especially peaceniks) start arguing over what ways to kill are okay and which are not.
That was just plain silly, Frank. Examining the ethical merits of various ways to conduct war can't possibly be shoehorned into the term "peacenik".