When witnessing the repeated mass-slaughter of civilians, I’d rather err on the side of acting than apathy. Otherwise people will use this excuse endlessly in future conflicts.
From a military perspective, Syria is meaningless. It cannot start World War 3. Of all the players invested in Syria, Russia is the only danger but even it does not care enough about Syria to escalate to this level. There simply isn’t enough at risk for them there. Russia is in Syria in order to expand their weapon-selling program. They aren’t defending personal assets.