She has been waiting for him. It is this image of her sitting on those stairs while the castle is coming down around her, that will forever haunt him. He knows he doesn’t have much time - he has made sure that Potter and Wesley are close on his heels and will be there to get her to safety. Right now her safety was all that matters to him.
She knows. He can see it in the way she looks at him pleadingly, as he raises his wand to her. She has always been to smart for her own good. “Please” she whispered. “Draco, don’t do this. I love you and you love me…”
He does. He has for a long time now. And that is why he doesn’t have a choice. She deserves so much better than to be bound to a war criminal.