inexperience? much like Saddam's use. if indeed he did use them at all before, ignoring all rebel groups/uncommanded actions of the syrian army
you say yourself he doesn't have a clean record, why would he risk it for literally no gain.
he gained nothing from this but a massive set back.
possible reasons are;
1) the your/nytimes opinion that he did it to look "unimpeachable/untouchable". a reach at best
2) hes a lunatic unconcerned with us/western/global interference, possible but unlikley as he's not an idiot/has russian advisors
3)some form of provocation either at the behest of Russia or inspire of Russia trying to draw them and the usa/natio into closer conflict to give him protection, potentially possible from either view point
4) the russian perspective it was a rebel/isis held mutations facility that happened to be storing chemical weapons, possible, but the question is known or unknown, no speciall attack was used so they may hav ebeen unknowing or they may have known such weapons where there and attacked conventionally in order to release and expose them at the cost of civilian lives.
the last option is the one i find most likely.
a knowledgeable strike on a chemical weapons storage facility. it leaves assad/russia blameless and victims simultaneously while offering a horrific propaganda victory.