If I stay he will be caged in pain for as long as he live. But if I leave him, I'll be the one in that cage.
He will be branded with agony if I don't hurt him. But if I stab him once, time will be able to heal his wound.
I am only human. I am selfish. I desperately wanted to enjoy the remaining time that I have. I desperately wanted to be by his side until the end.
But he is important to me. He means a lot to me. I understand him better than he understands himself. I know him better than I know myself. Most of all, I love him. I am in love with him.
That's why I'm willing to be the one in the cage and be the villain. Because sorrows for Chester, My Chester, is nothing if he will be free from that pain and happy in the long run.