If a barrel bombing campaign can accelerate the course of this war and put an end to it sooner, then I view it as justifiable. The US government pulled a MUCH MUCH nastier move in WWII, but in their eyes it was necessary to force Japan to surrender. This isn't that different.
A) yes it is MUCH different. This isn't 1945. The world has generally accepted that area bombing civilians is NOT an effective means of warfare, it doesn't force surrender, and the effect on morale is generally not enough to impact the fighters in the field.
B) just because it occurred then doesn't mean that it was okay. I'm vehemently opposed, morally, to the fire bombing and nuclear bombing in Japan.
I think you're fundamentally misunderstanding the way barrel bombs are used. They are not dropped on the front lines. They are not dropped on army bases. They are not dropped on formations of soldiers or lines of tanks. They are dropped on apartment blocks and buildings filled with civilians. They are PURPOSELY trying to kill civilians and destroy infrastructure in the pursuit of making opposition held territory both uninhabitable and unsupportable, to prove that the opposition cannot protect them, and to force them away from providing support, either material or moral to the opposition.
Reality of the situation is that he's in a pickle. He's losing soldiers and weapons on a daily basis, and the Syrian civilians are getting restless.
Of course he is. But your last bit really encapsulates part of his problem. His response to losing territory to the rebels is to INTENTIONALLY and as PART OF A STRATEGIC PLAN, MURDER THE PEOPLE WHO REMAIN BEHIND. That is 100% indefensible. It would be one thing if he was dropping barrel bombs (to little effect) on completely empty cities, i.e Kobani, parts of Aleppo that essentially empty, etc etc. And he has done that- I take zero quarrel with bombing the, more or less, evacuated areas of Aleppo. But they didn't use to be evacuated and they were bombed even then. What I really take issue with is "Hey, that city of 150,000 people that we owned up until like 3 days ago? Yeah, fuck them. Start dropping bombs randomly throughout the city as punishment for us fucking up and losing that city." That is where I take issue.
You're absolutely right, collateral damage happens. And legitimately, I say this with complete honesty- I do not begrudge the Syrian military for collateral damage. When you are TRYING not to kill civilians and it happens anyway, that sucks, and that's rough, but you were at least attempting to avoid innocent death. But when you're indiscriminately targeting an entire city, that's not collateral damage. That's intentional murder.