Why does that matter? What people lose sleep over or not can be due to totally trivial reasons or for reasons that are far removed from what the biggest threat facing them.
I have read a bunch of them and the massacres are clearly in response to resistance movements so they cannot be the root causes. The problem isn't a trigger happy government that just needs to get it's hand off its weapons and then peace sets in.
If anything, if I had to point to a root cause based on almost no research I would start here https://en.m.wikipedia.org/wiki/2011_Syrian_Revolution and not with some random massacre.
"According to the report by The New York Times, the crowd had been identified as civilians by U.S. drone operators based in Al-Udeid airbase in Qatar. The drone operators were reportedly stunned seeing the first 500 lb bomb dropped, followed by a second 2,000 lb bomb on the survivors; 80 people were killed including Islamic State fighters according to The New York Times report.[2]"
How on earth is dropping a bomb supposed to be a root cause? If you really wanted to point out root causes you would link to the most important massacres instead of expecting us to do your research for you.
I'm not dismissing its ecological affect, but to claim it's the root cause, I see as belittling and trying to erase what the people were actually fighting for:
If you're in the mood for harsh history lesson, there's a list of root causes here: https://en.wikipedia.org/wiki/List_of_massacres_in_Syria