My brother, I hope I’m missing something.
Well... such ugliness and evil in the minds of these dictators is not something anyone sane and decent really WANTS to understand, except that it is necessary to understand to try to limit that evil.
It is also something that cannot be easily / quickly explained, and I still have complicated tax filing to get to (ouch!), but, way over-simplified and generalized, it’s use of fear / terror, combined with sadism.
Consider: Assad may have been in position to defeat the rebels for now (we are told), but, he would be left with a majority of his population embittered and (too often) radicalized. Muslim radicalized, if you get my drift. In addition, the Saudis (etc.) are still justifiably (for the most part) opposed to an Iranian / Russian / Syrian “axis” in the ME, and will almost certainly try again to stir up trouble within Syria.
So, you have with Assad capability and experience (in use of chem weapons, demonstrated sadism, and extreme need to control / strike fear into the population. Couple this with Obama’s “red line” that Obama did nothing about, PLUS a large scale Russian propaganda machine to help cover for him. I would guess that at worst Assad thought that if “caught”, he might lose a few more aircraft & facilities, which the Russians would help him replace.
Or... you can just go with “evil” if you wish.