She was beautiful which was exactly why he couldn't have her.
She was pure, completely seperated from the world.
Madeline was often too nervous to speak her own mind, and that was why Alistair couldn't have her; he tainted people and destroyed them.
Maddie saved them by her pure innocence as the evil in them was chased away.
She could seemingly turn a bad guy in to a good one as they feared hurting or breaking her.
Breaking her proved impossible to Alistair as he had found a hidden pool of strength in her which failed at making him feel less evil or bad.
She actually listened to him, and didn't seem to care that heartbreak was involved in loving him when no one else could love him.
I love you had never meant so much.
Stay with me as you make me better than I was before, love.