It's natural to feel badly about the death and destruction in Aleppo. Assad and friends are surely guilty are atrocities, as are many of the rebels of whatever flavor. That doesn't change the stark diplomatic and strategic realities. The only viable military option was full scale invasion, subjugation, and occupation like Japan post WWII. Nobody had the stomach for sending in millions of troops and spending trillions of dollars. And that's fine by me. Also the West has little to no strategic interest in Syria, apart from simply wanting Muslim refugees to stay away. It would be a waste of blood and treasure to embark on a colonial military misadventure in the region when there was no obvious benefit to us, and it's not up to us to prevent them from slaughtering each other. 

Sadly, apart from "tsk, tsk'ing" from the sidelines, their really isn't much else Western powers can do.