Here's the situation, Yunzer.
There is a lot of propaganda against Assad in the United States, and a lot of it is being done by people supposedly on the Left. This propaganda is intended by the powers that be that are behind it, not necessarily all who believe it and spread it, to justify the United States using military force to remove Assad.
This has been the agenda of the Neo-Con establishment for well over twenty years- they told us in a paper they published twenty years ago. Clarke told us this was the agenda. There are many reasons they wish to get rid of him. A quick overview includes removing an enemy of Israel that supports Hezbollah and better control of the oil and gas of the Middle East.
For a while now the Liberal Interventionists are totally simpatico with the Neo-Con agenda.
Our job as Leftists in the US, who oppose imperialism and aggression, is to speak out against the US militarism. Speaking out against Assad's militarism is not our duty because we are not financing his aggression with out taxes nor are we justifying his actions with our votes. Our duty is to speak out against the aggression that is financed by our taxes and justified by our votes. (Remember even if we vote against those who end up in power, our votes add justification to them as being the legitimate leaders of a democracy.)
There are a number of reason to not join in the verbal attacks on Assad. (After all one might argue that yes it is our duty to speak out against US aggression- as you have done- and yet one still can speak out against Assad's aggression too. So in response to that it needs to be discussed why not to speak out against Assad.)
1) A lot of the verbal attacks on Assad just aren't accurate. There is no proof he used chemical weapons on his people, either a few years back or a few weeks back. Yet the propaganda machine to go to war against him has made these unproven assertions accepted truthiness in the US. If Leftists who oppose US aggression aren't willing to point this out, who will? There are other examples like this- for instance that the civil war is all his fault or that it is even a civil war. If Leftists don't question the narrative, then the narrative dominates and war making by the US becomes more likely. It is better to call into question the narrative when it might turn out the narrative is accurate then it is to not call it into question when it might turn out it is not accurate but simply propaganda. It's not like the Left is overwhelming the narrative. The reality is most people in this nation have no idea what we're saying and think the narrative is so obviously true everyone agrees.
2) Focusing on Assad's wrong doing tends to go hand in hand with ignoring the wrong doing of regimes working with the US and being actively supported by the US both militarily, financially, and diplomatically. It is not speaking out against all militarism and human rights violation. It is actually being silent about such stuff except when the object of this speaking out also happens to be the target du jour of the American Imperial Project and the current Bad Guy of the Week. The result, even when the Leftist adds he or she opposes US militarism, as you have, is to spread a narrative that is unbalanced and that creates consensus to support war. Every time Assad is demonized (even if he is a demon) is making it easier for the US to kill civilians in Syria and easier for the US populace to turn a blind eye on injustice done by US allies with US backing.
3) Every time the critique of American's injustice is coupled with the injustice of American's 'enemy,' it waters down the protest against the US. Again, since the only nation we as citizens have a duty to shape its behavior is our nation it is vital we not water down that work. Rhetorically we know that no matter how much one presents a viewpoint, to then say, "However..." and then present another one is actually negating the earlier viewpoint and only the later one really matters. So a rhetoric of "The US must not intervene in other nations or remove regimes with military force, however, Assad is a terrible person and not speaking out against his crimes is disgusting" actually is only saying the latter. That's what is heard and that's the effect. So we must simply say, "The US must not intervene in other nations or remove regimes with military force. PERIOD." If we don't do this, we might as well just shut up and do nothing.
4) Speaking out against Assad accomplishes nothing to change things there. The enemies of Assad already are killing people and blowing up things to remove him. The people who support him know more about what is going on than American Leftists blogging or commenting and if they even knew what we were saying they would at best simply ignore us or, at worst, see us as part of the enemy making war on them. So what does it accomplish? See points 1 through 3- it makes American aggression more likely.
So it is not supporting Assad. Leftists, except those who are revolutionaries and raise funds for violence abroad, are not supporting anyone abroad when they call into question the narrative against that regime. Instead they are opposing their own nation's policies.