What did the guy do?? If they're gonna keep him away from Seonwoo, why did they put him in...

I think they see him as a threat because he's conversing and getting unnecessarily close without need. ML literally said, protect him like I would, and this is exactly like how he would protect him

They had him join the party because he has poison skills, but that was planned before they had all gathered as a group and hadn't seen him react to Seonwoo.

I imagine the two of them are now instinctively picking up on the strange vibes he is giving off towards Seonwoo and now are perceiving him as a threat. Their top priority is keeping Seonwoo safe and this guy is glaring at Seonwoo and trying to talk to him instead of keeping on task. They gave him a warning once and he's pushing it.
They're high ranking hunters who would be able to sense that's something is off.

I mean when the party was first formed, Chorok and Taera already tried to distance Seonwoo from hunter Koo, and all the guy did was stare at him. And even Jehee let Seonwoo talk with people he just met before, so unless the guy had some criminal records, I don't get the reaction

All the staring could also be seen as him knowing Seonwoo before, which he actually did, so unless he had some past misdeeds, why did they conclude he was a danger instead of 'oh maybe he had some connection with Seonwoo'. I feel like there's something more, another reason why they acted like this

I'm saying that they have a higher sense of danger due to their profession. It's like animal instinct.
Someone could look at you totally normally and it not set off red flags, but the way this guy stares at Seonwoo is predatory and they can tell.
They didn't do anything when they were outside the dungeon to keep him away specifically. They were already clinging to Seonwoo because they were told to keep him safe.
But if you go into a dangerous dungeon and this dude is glaring at Seonwoo in a not normal way and not doing what he's supposed to be doing for the mission then that's weird.

When he first approaches Seonwoo he's trying to nudge into "protection duty" which is not his job. The healer guy says leave him to us (the people who already have protection duty) and go do the task you were assigned. He warns him to not "glare at him like that" indicating he's being weird in the way he looks at him.
He then DOESN'T stay on task and tries it again with even MORE noticeably bad vibes. If the man is being weird to a party member AND not doing his assigned mission task or listening to the person in charge that's really good grounds to kick him out.
Their gut instinct is correct and they have to watch out for danger in and out of their party.

No, they already did want to separate them when they were outside, and only did a basic introduction. I'm not denying they have sharp intuition, more so than normal people. But see, this is where it's a bit tricky, they shouldn't have let him in if he's a dangerous guy. A person won't suddenly change their nature so suddenly, so there would be signs of him being creepy even before meeting Seonwoo. Even if they needed someone adept at poison, they could've picked someone else. I still feel like there's something else about him that Taera and Chorok haven't told Seonwoo