It's more effective that way, but it doesn't really happen a lot bcs yangang can purify in a lot of ways (by simply touching them, by giving them purified foods, by simply being near them bcs he's technically oozing lights from his body, etc) but he only did kissing and more with ml.
I think the killer is a possibility because of the mention of a killer , in episode 7 I believe, where they talk about murders. If it’s not that then I think it could be foreshadowing.
"Sir, I'll tell you want I want".
Charr