Oh I thought that the ending was implied. Within the series Sangwoo told Yoonbum that if he was to ever die then Yoonbum must kill himself and Yoonbum agreed so the ending was fulling the promise. But I can see how some the ending may have been confusing. tbh I was confused too but I went to the comment section and this is the conclusion some people had.
even if stalking killing's ending was vague, it was heavily implied, the tension was real, the emotion was there. SK had a good ending.
Here. Here.. it's just like. Ah shit, here we go again. No tension, we all knew it was happening again because he set so many red flags (haha look im happy now, haha look im going to face my trauma, haha i thought this guy was gyuh wool). There was no emotion (the entire series was just flat like ??). and there was no character development. It was literally, ok the same thing will happen again.

That's some Killing stalking ending kinda bull shit.