There is a reason why nuclear, chemical and biological weapons are subject to specific treaties prohibiting their use (eg: https://en.wikipedia.org/wiki/Chemical_Weapons_Convention). These weapons' capacity to wreak havoc  far exceeds the comparable capability of conventional weapons. Most countries' self-restraint in the use of the former obviously accounts for the difference in casualty rates between the two (Assad's regime killed 1400 civilians in the town of Ghouta using sarin gas in 2013.) But I do not mean to diminish the horror inflicted by so-called conventional weapons. Assad has had great "success" employing these against his civilian population: between March 2011 to March 2016, the Syrian Network for Human Rights (http://sn4hr.org/) notes the Syrian regime and its backers were responsible for 188,000 civilian deaths or 92% of the total.  I don't know the exact split between chemical vs conventional deaths but SNHR claims there were 9 chemical attacks in 2017 so far.