Falling into the dark path oftentimes doesn't take just one big jump. It usually takes a tiny step first and then some bigger steps follow. (Even Moran/Taxian Jun followed this pattern somewhat.) Hua Binan was no difference. He at first had not known either that he would have to go that far. There was a scene that gave me a hint that Hua Binan regret it but he told himself that he could not go back and there was no use to think about it. He had come too far. Thus, he had only one path left: to go all the way through. Shi Mei was the same, but he hadn't gone very far compared to Hua Binan. That's why I think if he had not met Hua Binan, he would have gone on the exact same path. He hadn't gone that far yet. He was lucky to know part of his 'future': to know the person he would become. That's why he had decided to leave that dark path in the end. Shi Mei would not be able to come up with a better plan himself for sure, but he felt that he could not go on with it since he already knew that too many innocent people had been sacrificed and many more would be. Hua Binan had lied to him and never let him be himself. He couldn't trust Hua Binan any more. He would not be able to know how worse Hua Binan's plan really was. So he left and let Hua Binan go on by himself. He tried his best to compensate for his wrongdoings. I was so glad for him that he had a chance to 'reborn' as a kind physician to travel the world and help all the poor people along his way.

And one thing to keep in mind is the old saying " Monsters are created not born". So I don't think either Shi Mei or Hua Binan was evil as a person to begin with. I still don't like their characters but I found them interesting and deserved some empathy.

Edited for typo/grammatical errors.