So many hints and I still can't put them all together. Just finished watching episode 6 and I think it's pretty…
I also have the same questions as you, but the only reasonable answer I could think of for the first question is that, maybe he’s not hurting Sung Mo, because he wants something from him (like you mentioned, it could be his mom.) I think the stalker is killing people near him because he wants to show “if you don’t do as I say, they’ll all die.” Also, a question I have is, why did Jae In’s dad die when she told him she’s going to start finding clues. Why is he trying to “save” the real culprit??? So confusing xD
I honestly think he’s still suffering from the trauma. I also think he left Lee Ahn at the adoption centre (I think it was that??), because he felt a need to “hide” so that his “stalker” can’t find him again, which we saw recently (I think the end of ep 4), the stalker says “I found you.”
Maybe how Lee Ahn mentioned he hates people with two faces is related with the poster of a mask when Sung Mo was dropping off Jae In?
Also, I think Sung Mo knows something. I think he knows why Jae In’s dad was chosen/forced to be in jail? Whatever it is, I don’t think he’s the antagonist in the end, but somehow, he knows the antagonist??
Celebrity 
Also, I think Sung Mo knows something. I think he knows why Jae In’s dad was chosen/forced to be in jail? Whatever it is, I don’t think he’s the antagonist in the end, but somehow, he knows the antagonist??