It is a reasonable red line: if US personnel are explicitly entering the war, Russia and the US are now explicitly at war. Being at war means the US becomes a target, that Russia could, say, blow up THAAD in Israel and destroy the "secret" US base and Daesh training grounds in Jordan and Syria. At some point, nukes become relevant.

Do you think war between Russia and the US is a good thing?

Yeah, but shooting down the missiles personally would turn it into a direct war