They are the same ones who witnessed them leaving together, and I think that definitely seems like the obvious reason why he ambushed them, but (going by previous chapters and plot twists in this work) I also feel like it might, just maybe, be *too* obvious to actually be the case.
I'm very curious about what happened to the second cop, for my part (since Seo apparently only kidnapped the first one). From the looks of it, he was attacked from the back and Seo *covered his eyes*, so it's quite likely he didn't actually see Seo's face. I feel like whether the second cop (who wasn't kidnapped) was actually killed, or maybe actually just left there unconscious or something along those lines, should tell us a bit more about Seo's likely motives behind ambushing the cops in the first place - if it was really just because they were witnesses, or if maybe the kidnapped cop *might* actually have some proverbial skeletons in his closet as well.
The newly introduced red eyed guy is the tech guy Ah-In took his spy glasses to for fixing and data retrieval. The tech guy had told Ah-In that it was too damaged to be retrieved but clearly that’s not the case since he has it right now. It’s very likely that the tech guy held into the video solely for the purpose of blackmailing the killer. I’m assuming this is to get money to either escape his current abusive situation (which he wouldn’t be able to do if Ah-In turned in the video) or he’s just as fucked up as the killer and is enjoying this. The killer is still being whack as fuck and just killed a cop and captured another most likely because they themselves are witnesses should Ah-In die/go missing or they suspect that someone in that area is responsible for the missing cases. And Ah-In just missed a chance to escape because of ‘love’ so now he’s got the baggage that is his fuck buddy who’s probs going to leave his ass as well as emotionally hurt Ah-In if they both make it out alive (I don’t mean that his fwb shouldn’t be saved or whatever but rn Ah-In doesn’t have the luxury to think of others when he isn’t safe either)