Ending seemed pretty good, although not very surprising.

not spoil future plot points for unsuspecting eyes.

Hate to break it to you, but there may not be many future plot points now that the finale has aired :)

SPOILERS:
Overall the series was good. The ensemble cast was fine but Kwang Soo still has too much Running Man DNA in his acting. I am still impressed by the quality of Korean child actors as Yul (Ahn Se-Bin) did a wonderful job in a difficult role. The writer also gets a nod for having a coherent script.

The prime suspect is revealed in Episode 7 as Yul’s violent father. By episode 4, he was my choice as the killer since he had access to the apartments, bought or used the things from the MS mart and lied about his wife being at the hospital. He wanted to get his wife’s cell phone back because it has incriminating evidence. Yul was smart enough to hide it at the one place she felt safe, MS Mart. But the one problem was the motive to kill the other young women. My initial theory was that he did not want Yul to get close to any other woman so she would not forget her mother, who had left him. That would have been a sad pretext for killing the women. When it was clear that was not the case, the story turns very dark in that he was a man who liked to be violent to control women. The writer did a good job of having enough clues to point a finger at just about everyone in the cast (which was a good thing) and there was very little filler. Like all drama happy conclusions, the ending of this one was fine.

The short series format did not give the director time to evolve complete back stories for the main characters, especially the romance between Dae-Sung and A-Hee.