Law enforcement never went after Tom Hagen, who after all was someone who was privy to all of Michael's plans to murder people, etc. He had no buffers, and he knew enough to put Michael away forever.

There is no attorney-client privilege where the client is telling the attorney in advance that he is going to commit a crime. If someone tells an attorney "Yesterday I killed so and so," it is privileged and the attorney cannot be compelled to testify. On the other hand the "crime-fraud" exception to
privilege is if the client says he is going to kill someone,
the attorney is OBLIGATED to rat the client out. Hagen knew Michael was going to kill Sollozzo and McCluskey; that Michael was going to "going to meet with the heads of the five families" and that Michael wanted Roth's "plane met in Miami." He also activley participated in the blackmail of Geary, which happened to involve a murder. So why was Hagen never targeted by law enforcement?

Could it be that Michael's often public and frequent banishment of Tom from "certain meetings" have been a ruse to make people have the impression that Tom was not, as Sollozzo wrongly believed, "on the muscle end of the family?"