Do Kyungsoo hated his reflection. 
Whenever he looked in the mirror, he saw purple and green, reflecting the painful and disgusting life that he led. With no mother to tend to his wounds, he would try to cheer himself up, holding back the urges to just take a knife and cut out the source of his pains - his lowlife father. He just needed someone to tell him that everything was going to be okay, that everything will get better soon....but who would? He had no one. 
And when his final strings began to snap - to shred away his decaying sanity - he met him.

Kim Jongin. 
With deep, soft eyes and delicate plump lips, Jongin would often come over and give the owl-eyed boy the words he needed to hear - the words that would keep Kyungsoo from ending his father's life, and even his own. The beauty of Jongin was that he was everything Kyungsoo needed, becoming his only source of happiness and the only thing keeping him from dirtying his innocent hands.

And Kyungsoo can't let go. 
He won't let go.
With his final strings wrapped tight around Jongin, he plans to keep him there forever. 
Because losing Jongin means losing his only comfort, friend, and lover. 
Because losing Jongin means losing his only source of sunshine in the dark world he was forced into.
Because losing Jongin means losing his heart and mind.
Because Jongin is his. 
And Jongin is his sanity.