"Have you ever loved someone so much, you'd give an arm for?

Not the expression, no, literally give an arm for?

When they know they're your heart

And you know you were their armour

And you will destroy anyone who would try to harm 'her

But what happens when karma, turns right around and bites you?

And everything you stand for, turns on you, despite you?

What happens when you become the main source of her pain?"