So they had him in their sights presumably they saw he was armed. The question is why did they wait until he shot before killing him.
Maybe they were told over their comms not to fire until the assassin fires. Even then it took them 16 seconds to fire one shot at the assassin after he fire his first shot.
I saw last night where they show the sniper’s head rise up from the scope once he heard gunfire. Not sure if that’s the very sniper that shot the shooter dead or not as you don’t hear the sniper’s rifle.