My teacher sucks

I wrote my composition so perfectly and only write nonsense in the last paragraph and thanks to that,I failed instantly?

The story line is about a robber robbing a bank and is attempting to escape.

Here is the last paragraph

The robber tried to escape with the loot. As a security guard,I have to stop him from escaping.
We are forbid to use firearm unless it is really needed so I only can use force to stop him
Suddenly,The robber took out a AK-47 from his backpack and begin to fire pot shots in the air to frighten the crowd as well as making a swift escape. I have no chance of winning him if I tackle him alone. I pulled my Desert Eagle .50 and wonder if it will be all right to use it. When all hope seem to be lost,I immediately took aim with great accuracy with my Desert Eagle .50 and shot the robber in the head.
“Head Shot!” scream the crowd as they applauded me for my heroic act of saving the day.

(0/100),inappropriate story line.
What the hell….
My teacher don’t know the art of writing