When it is used as a weapon, rather than as a means of camouflage it is a chemical weapon.

That is not correct.
The definition is to do with toxicity.
That is how weapons defined as chemical cause casualties.

When used as a weapon it is an incendiary, as is napalm.
They cause casualties because they burn.

It should not have been used in Gaza.
IDF accepted that.
The local commander was in error.
That is why I would not defend its use there.
If I had a son or daughter among those troops and they died for want of screening I would might feel differently.
If I had to explain to grieving parents, wives and orphans why their loved ones could not be screened from their enemies I might feel differently.

The situation arose because Hamas made their positions in civilian areas.
That was a war crime.