(Spoiler below!)
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You're right for thinking that way, cos I also thought that until I read the novel. Jehee actually explains why and apologises to Seonwoo for treating him poorly.
He said that he kidnapped Seonwoo and put him under a slave contract as an efficient way to learn his personality, behaviour patterns, and likes and dislikes, with the intention of regressing afterwards. Once he confirmed Seonwoo would be useful to him, he planned to regress and then build a more natural connection with him, effectively erasing the memories of his aggressive approach in the current timeline.
He also mentioned that this was how he had always operated, which is why Taera and Chorok are extremely loyal to Jehee, because he always found the most efficient way to gain people's loyalty through countless iterations and attempts. Remember in Chorok's flashback of how he said it was miraculous that Jehee found him at the right moment; this is why.
But after learning from Seonwoo that his regressions had ended, and that even if he somehow regressed again he might never see Seonwoo due to him being an anomaly in this world, he deeply regretted treating him that way.

I wonder why Jehee couldn't be like a normal person and just make an appointment at Seonwoo's Clinic(?) He could have made one under a fake name or call and ask if the appointment could be done at Radish guild.
Why kidnap a person who can help you with your problem(?) Kidnapping is a crime and is a bad rep for the guild if that came on the news.
Shouldn't Jehee be treating Seonwoo very well so he'll help him? Mistreating an employee is also a crime.
Also, Jehee likes Seonwoo but he doesn't really treat him well most of the time. I mean e.g. Forcing Seonwoo to do what he wants or when Seonwoo disagrees with him by manipulating his mood so the status effects drops, which causes Seonwoo to suffer. Seonwoo then has to do anything so Jehee will lift up the status effect.
Jehee only treats Seonwoo well during their first encounter after he regresses because Seonwoo died.
Don't get me wrong. I'm still invested in this manwha despite my complaints. I know it's just fiction. I just don't really understand why Jehee did what he did. Things could have been done differently. ( ̄∇ ̄")