Thinking about this some more, I don't think they even know that he's psycho like that. I also assume they don't know that he killed a civilian before he showed up for the meet. He probably acts different with gangster associates. Notice how when he comes to party he's joking with the crew and is almost mild mannered.
Then a fukking Dancing Business Tyc00n shows up and interrupts his business. He gets annoyed and then Nucky patronizes him and makes him wait.
So then Nucky comes back tells the crew that they're gonna have to get the booze from Rothstein now. Everybody is mad, newcomer (Rosetti) who wanted to start business with Nucky now has bad timing and is furious that he has to settle with Rothstein. So he loses his facade of composure and flies off the handle and disses everyone.
So Remus is like "chill out we all got the same news brah" but since things didn't go according to plan for Rosetti he doesn't give a fukk now. If Nucky didn't brush the crew off to Rothstein I bet everything would have been gravy but since the deal went wrong that makes Rosetti thinks he got the shyt end of the stick so he turns into an adversary.
I believe Nucky knew what happened because he saw the front page of the paper with the dead dude on the road...and then when he met up with Rosetti, he asked him about his trip & then sarcastically mentioned the area where the guy was killed.Maybe. I dunno. Like I said it's not a big deal to me it just kinda threw me for a loop when he spoke to Nucky at the party. His intro scene was kinda over the top so I had him pegged as some kinda wild card psychopath.
Fred.
I think he was pretty much hinting that he knew Rosetti killed the dude because that's the route he took to get there & because he's a loose cannon.