I can give some spoilers if you want :D
GIVE MEEEE PLEASEEEEE *zombie sounds*
Spoilers below:
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So you were right about the fact that the blond guy is the system administrator. But he's no villain. He's the ML lol. For the past timelines, he's been trying to prevent Seora's death (but failed. Not his fault. It's Seora who walks towards death itself). And while system administrators are a pretty scary big deal, our ML is the best and cutest system administrator! I don't know if she arrived yet in the manhwa (I read the novel, I want to wait for more manhwa chs before reading this), but there's a returnee character . She disappeared inside a Gate, where she actually went to another planet for 100 years. There she met another person who was that planets system administrator's contractee but that SA(system administrator) wasn't very nice. So while our ML, the Earth's SA, had the potential to be evil, he's not. Also he knew FL's mother. He had a bet with her which he lost. I forgot what the bet was exactly.
Also SA and Seora met/conversed once when Seora's mom died. He was going to give the inheritance of Seora's mom to Seora and her brother (which was powers). Seora at that time rejected because she didn't want to be strong and be shackled and all (trauma) so she forgot about that. Her brother on the other hand, accepted the inheritance and hence became an S class.
Ohhhhhh now that makes senseee...niceee thank youuuu
My theory is that the system administrator might have been the blond guy right? So from what the 2 characters like the returnee and the shaman have stated, admins are like a God and as per what the Shaman have shown thru her fears, the admin is somewhat so frightening that it can end the world. So my guess is that, if the MC is dead for the past lives of the Sejun and with the Shaman's hints that the admin can do worse, the admin might have been the villain that Sejun, the revived, might be vying to defeat in the labyrinth. My guess on why the admin was the villain might have been connected to the MC's deaths