1. These were US missiles.

2. Russian government officials stated they would shoot them down.

3. Russia had NOTHING to lose by shooting down missiles in defense of an ally, e.g. no political risk.

4. Russia had everything to gain by demonstrating the capability to deter a NATO strike, especially given the context of using Syria to advertise its weapons.

Israel is a separate issue, if you want to discuss it. Very nice of Israel to get Russia's plane shot down by hiding behind it.