As the audience we know that he wasn't a seasoned combatant. He was an asthmatic kid that had only recently been handed a gun. This gives us the incite to realize that Herschel's reading of the situation was the correct one. He was a scared kid with a gun in his face who had no plans to "take advantage of the situation". If anything he was scared into inaction by the gun pointed at him.
The asthmatic kid and the person Carl shot were two different people. For some reason several people on different forums are confusing the two.