Civil Wars are hideous and all moral judgments about Syria are on shaky ground
Take U.S. history. In the American Civil War about 700,000 men in uniform and 50,000 civilians were killed on both sides. The destruction off Atlanta by Northern troops reminds one of the destruction of Aleppo without the slaughter of civilians. Assad has been brutal but the various opposition forces (among them ISIS) also committed crimes. It is difficult to make absolute moral judgments. As a footnote US population right before the Civil War was 31.5 million. Syrian population before the present conflict was 22 million - comparable figures.
A revered American president ordered the destruction of Southern cities. Thousands died. He had his reasons, but there was no United Nations to condemn his actions.
In the South the government was begging France and Britain to intervene on the Confederate side. It came to nothing except for some economic support and the construction of some naval assets. If a modern-day Turkey had intervened on the side of the South in this conflict or if France and Britain (read Russia and Iran) had come in on the side of the North, things would have gone much faster
It is difficult to judge which side in the American Civil War was nastier. Focus on today. Now that the Syrian government is regaining control of its territory with the help of the Russians and the Iranians, it is very clear who possesses and uses the means to destroy the opposition - at the expense of the civilian population.
All of this is to say that passing moral judgments in this conflict is hazardous. Too many factors are at play, including the foreign powers that are jockeying for position in a post-war Syria. The war will end in a terribly bloody way. Unless a federation of government, Sunni, and Kurdish territories is established, there will be a long campaign to reunite the country. And any outside intervention, no matter how well intended, will only extend the conflict and increase the number of casualties.