"
I'm an angel with a shotgun,
Fighting til' the wars won,
I don't care if heaven won't take me back.
I'll throw away my faith, babe, just to keep you safe
When he says "I'm an angel with a shotgun, fighting till the wars won," it means that he's a good guy fighting for the person he loves the most.
When it says "I don't care if heaven won't take me back. I'll throw away my faith, babe, just to keep you safe," it means that he's willing to do whatever it takes to protect the person he adores the most, even if he won't be thought of as a good guy by the end of his war.