Consider;
Two bullets, one each in the head of assad and jun and how much different the entire world would be ... instantaneously.
‘How about he’s just another sick bastid like the rolly poley in north Korea.’
I have seen that argument made persuasively. Dictators tend to become paranoid. Saddam Hussein, for example, was hopeless paranoid—but Josef Stalin had him beat by miles. Paranoid people do not act rationally.
Assad was likely worried that Trump and Putin might forge a good working relationship. That would bode ill for Assad. The chemical attack has driven a wedge between Trump and Putin. Perhaps Assad anticipated that this would happen, or else he just wanted to throw a wrench in the works. Either way, he appears to have accomplished his goal.