Assad is a dictator who took the reins of power after his father died. His 'election' was as much a sham as Saddam's or Kim Jong-Eun's.
Mummar Khadaffi was no saint either. Both were typical 'strong man" types who kept their respective populations in line through iron fisted policy. I'm not debating that, nor am I lauding it, I'm just saying with those populations in those areas, it is effective.
One of the effects of the "Arab Spring" series of destabilized governments was the wholesale slaughter of Christians in those countries. Something which was not happening in Syria.
We don't particularly like him, he is not a 'nice guy', we may not agree with how he came to power, but he is their President. We have more reason to see Kim Jong-Eun as a threat than Assad, but we're playing paddycake with the North Koreans, while Obama was all hot to trot to topple another Mediterranean regime. Recall, the American People pretty much just said "No.".
Between the behaviour of the US State Department and decisions made in re the disposition of American Troops in Iraq, I believe it may be possible that Obama & Co. were trying to enable IS, and eliminating Assad would have worked toward that end. The Russians stepped into the relative vacuum we left by not supporting Assad (not saying we should have, nor should we have even been involved, much less talking of opposition), and improved their hegemony in the region while using that to operate against IS, which will eventually threaten the relative stability of the former Soviet 'stans. Talk of deposing Assad left him little choice but to side with the one group who could promise some semblance of 'protection' from US intervention.
YMMV.