A common thread amongst the sort of "left" that I criticized in my LARB peice, is asking "what incentive would Assad have to bomb his people with chemical weapons just as he is emerging victorious from the Syrian Civil War? It's the one atrocity that has prompted threats of intervention from Western powers."
First, I think this rhetorical question (it is really deployed as an argument against anyone arguing Assad is responsible for this) ignores the fact that the Syrian government has used chemical weapons before in this war and the US and its allies did very little in the way of direct military strikes aimed against the regime.
Additionally, there is a simpler answer to the question of why states use indiscriminate force: It works. See this piece of good (and controversial) work from Professor Jason Lyall who looks at Russia's use of indiscriminate force against Chechen villages. It is too early to know who is responsible for the latest use of chemical weapons and Syria, but there is no reason to think that the Syrian government wouldn't be a prime suspect.