Justified or not, Michael felt Tom was loyal, but not a good consigliere. From Michael's viewpoint, Tom failed to see that Carlo would betray Sonny, leading to Sonny's death and Michael's forced ascendency as Don. Kay had an abortion on Tom's watch: even though he forbade her to leave the compound, she must have contrived to have a doctor whom Tom approved perform it. Then Tom failed to find out that Frankie Pentangeli survived the attempted murder. "Our people with the New York detectives said he was half-dead, scared, talking out loud..." Duh, Tom! How come your people with the New York detectives didn't tell you that before you allowed your one and only client to perjure himself five times before a Senate subcommittee?