He was forcefully taking her back. Against her wishes. Literally grabbing her full force and taking back. He's just a bodyguard, he doesn't have rights like that to decide what's better for her, what does he think he is? An equal? A brother? He's just a servant her family has bought
He's still a human being, don't talk so dehumanizingly about someone like that. I agree that him trying to forcefully take her back against her wishes wasn't okay but it's not okay because she's also a person who's fully entitled to live her life the way she decides.
If you want to simplify it down to just being a servant her father is the one he takes orders from so he is just doing what he was tasked with
yall cannot be srs acting as if kotaro wasnt completely reasonable just trying to get her to come back. If any of you guys would readily just trust this rando dude that tried to poison you or whatever, none of you would make good bodyguards be real.