Attacking a hospital is not a warcrime if the hospital is used for military operations.

It is, you have the right to defend yourself if the opponent is attacking from inside. But no even if the enemy is doing a war crime by using non-combatants as protection you are not allowed to engage them in that situation. You are allowed to inspect a hospital or ambulance to make sure the enemy isn't using it to transport or hide munitions. You are not allowed to attack anything marked with protected signs, such as the Red Cross, red crescent, or red diamond. If you as a solder break these rules, even on a direct order you are still personally responsible and might be judged in national or international courts. In practice, it might be hard to get soldiers to stand trial for these crimes. But it has happened. All my education in this has been in Swedish from the Swedish armed forces so some translations might be wrong.