Tom Hagen profited even without Michael being killed. Michael makes Tom de facto Acting Boss while he goes with Roth to investigate the shooting. This could allow for more wheeling and dealing with Roth in the background. Who better to help set up Michael with the Congressional hearings behind the scenes than a guy who knew the legal system better than anyone in the Family? Fredo would be useless to the Congressional Hearings unless he himself had testified.

Fredo wasn't in the loop when it came to big family things--He couldn't tell you that Frank Pentangelli was Michael's Capo in New York and Willie Cicci was his top Soldier. Someone like Tom would know all of this.

Tom Hagen was bitter at Michael as shown in the scene where he's contemplating moving away, out of the Family, without even telling Michael. He has no emotional connection to Michael outside of his vestige of Family Loyalty to Vito, which probably fully died when Mama died.

Fredo was weak and stupid and probably the extent of his treachery was giving out info which hurt the Corleones. But he didn't know it was going to be a hit. I think, even as bitter as he was and as naive as he was, he knew opening the drapes would mean a hit, and probably wouldn't have betrayed Michael to death.

Fredo made for a good scapegoat. The only guy who was actually losing money, influence and a place in the new Family business model was Tom. He was being left out of meetings and really had no place in the new Family and was losing money and only regained power and authority when Michael was nearly killed.

He was also the only guy Michael didn't suspect at all and the only one Michael fully trusted and thus didn't investigate besides Fredo.

Fredo wasn't strong or tough enough to kill the would-be assassins, especially so quickly. He couldn't even bring himself to hit his disrespectful wife, much less coldly and brutally take out two hitmen rapidly. And the assassins weren't hired out of New York...Who could hire assassins, not from New York, and also have the toughness to have them killed cooly and quickly but also the fear of being discovered? Who would have the means and intelligence to have the murder of the would be assassins covered up?

I'm sure Tom had men working under him directly, if Fredo did.

Consider how Tom handles the murder of the hooker. Cold, fast, with no real way to connect it to himself.

Keep your friends close but your enemies closer. Tom knew that lesson as well as anyone. Michael's trust in Tom might've been too blind to see it.

I also find it suspect that Tom has no emotion toward the unspoken plan to have Fredo whacked. Fredo was as much a brother to him as Michael or Sonny. Michael at least had guilt. Tom didn't seem to have any emotion over it, or at least try to counsel Michael not to have Fredo killed.

He showed more regret about having Frank Pentangelli kill himself, and more apprehension to Roth being killed, than he did to having Fredo killed.