Rescued

I saved a dog's life today— Her name is Mollie. 
She's this black and white four year old healthy sheltie with a heart-shaped face and sorrowful brown eyes.
 Her owner's no longer want her. They've left her at the vet to be euthanized.
How does anyone do that? What could possibly be wrong with her?
I have a horrible feeling she's terrified of everyone and everything.
I can't change her past.
I have to help her—teach her to trust and love.