His Hollow Heart by Rachel Leigh 

Bella: A pact made. Two orphans who vowed to never part. I was selfish. For once, I had to be. The last time I saw Callum he was standing at the dormer window as I sat in the back seat of the car. His dark eyes foreign to me.
The hatred that radiated from his glare shot through me with more force than a silver bullet. In a matter of minutes, he had become a stranger. No longer my best friend.
He was now a boy with a hollow heart who was headed down a dark path. A path I knew I’d never cross again. Or, so I thought… Read more.