I will never, ever understand why chemical weapons would be used in this conflict.

Last time chemical weapons were used in Syria, it was terrorists. The US tried to blame it on the Syrian government to justify our illegal war there, but it was terrorists. Terrorists like al Qaeda in Syria, a group that the USA actively supported once they changed their name to "al Nusra". I've also seen reports that the terrorists may have been from Turkey and that the gas they used came from the USA. There's a lot of conflicting reports on the matter since many media outlets are straight-up lying with regards to what's been going on in Syria these past few years.

I don't know much about this recent event, but my point is that there is more reason to use weapons than you are considering. Sometimes the terror of the attack is worth more than actually killing the people with the attack. It's what makes terrorism effective at bringing change to the world. Look at the way the world changed after a few thousand civilians in New York were killed in 2001. Those deaths aren't what changed everything, it was the terror that those attacks instilled.