Roselli was killed for a multitude of reasons. Not only was he a Giancana supporter but many suspected (correctly) that he had ratted out the Detroit and St. Louis families' Vegas operations. What's more is he had been slowly losing prestige ever since the Capone days, even. In my opinion, just speculation, Accardo disliked his high profile. There was an incident a decade or so before his death when Roselli told Murray Humphreys wife a story of a murder he committed. Very distasteful but also flashily distasteful. Humphreys made sure Accardo knew, and I doubt Accardo forgot in the years later.

No matter what factors there were in his death, I don't believe someone who schmoozed with celebrities and talked Cosa Nostra affairs to other mobster's life could be trusted to keep a low enough profile to co exist with the post Giancana, low profile regime. If he hadn't been killed the way he was he just would have been killed a year later. The Outfit had no use for people like that. I also doubt the Outfit appreciated the fact that Roselli got them involved with the CIA at all, and now that investigations were being prepared, why give him the chance to tell the government anything? IMO that was Aiuppa's thought process. And Santo Trafficante's thinking was obvious. Regardless of if you believe in a JFK conspiracy, there were still questions about mafia involvement in a Castro assassination plot, and Roselli could expose Santo as a conspirator. Of course he wouldn't want to be questioned.

Basically I believe the two main factors were the high profile Roselli had in the era where a high profile was intolerable. The second was Roselli's involvement with the CIA, a link that didn't need to be uncovered Bevause it would only lead to scandal and media attention. Rosella also ratted out Detroit and St. Louis. Take your pick..