I'm not as versed in the situation as I should be to offer an opinion (and I don't want to do the research, too depressing) but I wonder if, as in the prelude to WW1, the "powers" haven't put themselves in a position from which there is no good road out. There's no path to peace, no compromise that will convince everyone to stop killing.
Sure, everyone could just stop killing people. Period. Without preconditions, without diplomatic gain, without accomplishing anything, but what would be the point? Sadly, that's not the kind of things that "powers" do.
Obviously we don't want this discussion to become political, wrong forum for it. But this is not a military situation. In my mind there are three kinds of killing: murder, where someone kills because they hate them or want them out of the way; war deaths, where death is a too-often necessary by product in the pursuit of political goals; and terrorism, where people are killed indiscriminately to create fear that advances a cause. The difference between the last two can be subtle and plenty of people are eager to quibble. But there is no military reason to bomb Eastern Ghouta, killing all those civilians doesn't appear accidental or to be aimed at any military or political goal. Is it payback? Is it to improve governmental power by showing people what happens when you resist?
Turning our heads would be bad, but what else can people do? Throw the full support of the west behind Islamist rebels and end up in a full-on confrontation with Russia? Switch sides, accepting the accompanying loss of diplomatic credibility, to help the Assad regime restore order, winning peace but supporting a brutal dictator? Pull out and hope that Russia and Iran work out a peaceful solution? At least that way we could blame them for the whole thing. Donate money? International aid organization are banned from the places that need help most, because that just make sense.
I'm sure some people are turning their heads. I don't think I am, but I'm not doing any more good than those who are. Let's just be thankful that, at least so far, we're looking at low triple digits of dead, and not high quadruple digits like Srebrenica.