He definitely has to feel some sort of guilt. I can’f say for certain since at first he seemed like a manipulative person. But after seeing his expressions and memories, it doesn’t seem like he hated the OG I-heon (maybe even wanted to be friends or more) so I don’t think your theory is wrong tbh
Sekyung definitely feels guilty. And I think he's completely aware of it too. The fact that he spend so much time looking for I-heon to the point where he got sick is kind of an indication that he's aware. It make no sense for him to go to such length without knowing the reason why he's doing it. Especially when he has a calculating peronality.
This is just my thoughts, okay.
But I think that ml here is feeling a sense pf guilt for not being able to be there for the OG I-heon. I don't know if he realize it himself but there's a clear indication that he does and he is feeling it. And so, he turns those feelings into suspicions and vents this to the now MC I-heon. He clearly hates himself for it but doesn't know how to express these emotions since he grows to this age behind a mask, a pretender who must not express thoughts that's detrimental to him. He is used to it and has forgotten this feelings but due to the MC I-heon and the OG I-heon, this feelings resurface and turned into a thing called "curiosity".