When someone tells me they have my back, this is what comes to mind:

With knives in both hands, you are surrounded my the enemy. I can hear the promise echo,
"I've got your back."
From then on, I know that I can trust you. With us fighting back to back, I know that we will get through it.
"Don't worry, I won't let a knife hit you in the back." I promise.
When I make that promise, I am saying that I would give up my life protecting your back. No knife will hit your back as long as I'm alive. We fight together as one.

But someone close to me decided to change the ending.

Instead of defeating the enemy, I felt the knife pierce my back. With shock I fell. I turned to see my friend holding the knife that had ended me.
Thankfully, I rose stood back up after they stabbed my back.
If we are ever back to back again, I won't trust him.

If he comes back, he will pay.
But from now on, I will be alone.

In this horrible fight called life.