If your country is invaded, would you care about Geneva convention?

You make it sound like it is intentionally done but if the shooter is far away/has poor accuracy and the soldier has body armor, chances are he will be incapacitated but not killed. After that the question is: is more morally ok to killed the downed soldier or wait for his unit who is still and active aggressor.

Yeah, intentionally is not allowed, you are supposed to try for a clean kill. Doing so is as you point out difficult.

Interesting I would have guessed the opposite. Why?

Because the hurt suffer a lot. Getting through life missing a leg is not pleasant. Also dozens risk their life saving a down but alive friend, while you will leave the dead until it is safe