I think this is something that people are missing.

Because Russia is now experiencing terrorist attacks, that means that they're justified in defending themselves. ISIS and the Syrian rebels are a direct and immediate threat to Russia. Giving them justification to increase their military actions in Syria.

Then Russia (working with the Syrian government) is accused of using chemical weapons. If Russia does ramp up its military actions, they'll appear as if they're big bad murderers who are aggressively killing civilians and children instead of defending themselves.