Sure, his regime might have been the most transparent or the most democratic, and yes he was authoritarian and repressive and yes he has tortured and killed many. I don't for one moment pretend the Syrian President is a saint or infallible. HOWEVER, this is not an ideal world. Realpolitik dictates that currently our interests in the West (not the interests of a narrow circle of belligerent establishment interventionist hawks) and the interests of the Syrian people are best served by Assad defeating all non-government militants in Syria (excluding perhaps the Kurds if they are willing to cooperate with the government of Syria) and remaining in power for the foreseeable future. Let's get something clear, the "moderate opposition" which the US and UK and French governments boast about arming is a fantasy. The most effective rebel groups and the ones which hold the most territory and the most important strategic areas of Syria are all ideologically fundamentalist Islamist, most follow the Wahhabi/Salafi version of Islam and some of them are the cousins of ISIS (such as Jabhat Al Nusra which until very very recently was affiliated with Al Qaeda). The "moderates" have diffused into a amalgamation of sporadic, chaotic and ineffective militias.
Very little is left of the original Free Syrian Army we saw during the beginning of the uprising, most FSA soldiers have defected to more extreme groups like Ahrar-Al-Sham. Even Tony Blair, one of the most shameless interventionist hawks (who has single handedly done more helped destabilise the Middle East then any other British PM in living memory), had one of his think tanks admit that if ISIS were to be completely defeated, another 65,000 jihadists (many of them foreign fighters from here in Europe) could comfortably fill the vacuum left. And Blair wanted to bomb Assad by the way, so there's no bias against the rebels, quite the opposite. We have to stop our government from helping to finance and arm what are quite frankly terrorists and head choppers.
By giving them Kalashnikovs and rocket launchers, we make the civil war much worse. Not only do civilians suffer, but citizens here in Europe are prone to more attacks by jihadist sympathisers energised by what they see in Syria, sometimes even trained or directed by groups in Syria (mostly ISIS). If the Assad regime were to fall, the competition for power would not be between proponents of liberal democracy or different economic systems, the battle would be between the most efficient way to chop the heads of infidels. The security consequences for all of us living here in the UK and Europe would be infinitely worse than it is now, with no counterweight to oppose them, organisations which are ideologically dedicated to killing Westerners would be able to use Syria as a complete safe haven to plan more and more deadly attacks. This would allow xenophobic political parties to prosper in Europe and could even mean that our own troops have to intervene at a future point to clean up the mess we helped to create.
Recently an 11 year old boy was beheaded by a rebel organisation in Syria, and the group was in a US list of approved groups to receive military aid! Given the close relationship between the UK and US government, I have no doubt that MI6 was also helping to send them guns. Assad is the only person who can defeat those forces, and for all his authoritarianism, he is a secular dictator who does not persecute people based on religious identity. We cannot allow our taxes to help turn Syria into a failed state like Libya or even Iraq. If Assad goes, it must be by true moderates and democrats, but we cannot aid the most anti-democratic militants and claim to be spreading democracy!
Blair think tank report:
http://www.tonyblairfaithfoundation.org/religion-geopolitics/reports-analysis/report/if-castle-fallsBoy beheading article:
https://www.theguardian.com/world/2016/jul/20/syrian-opposition-group-which-killed-child-was-in-us-vetted-alliance