Mark Felt joined the FBI in 1942 and rose to become one of the most powerful men in that organization, in fact, an associate director, to which upon J. Edgar Hoover's death in 1972, Felt had hoped that he would be appointed the head of the FBI to which his credentials to become FBI director were solid, but instead Nixon appointed Patrick Gray an outsider to the FBI as the Director, probably in the hope that the new Director would be more Nixon's man, rather than someone picked internal to the FBI. Soon thereafter, just six weeks later, the Watergate break-in occurred, to which the intrepid reporters Bernstein and Woodward, eventually were to interact with a very knowledgeable inside source, who later became known as "deep throat", that provided to them guidance as well as important confirmation of information, of which many of the materials were known only to the FBI, conducting their own investigation of the Watergate break-in which included as stated by Woodward: "… access to information from the White House, Justice, the FBI, and CRP [Committee for the Re-election of the President)."
This means that Mark Felt, who admitted to being the inside source for Bernstein and Woodward, thirty-three years after the fact, was working for the FBI at the same time as he was leaking, confirming, guiding, and providing actionable information to the Washington Post reporters, a clear dereliction of duty, to which if Mark Felt really believed that he had documentation that needed to be presented for public discourse, the most honorable course to have taken would have been to present such to a grand jury, which he did not do. Instead, Mark Felt is on record, repeatedly denying that he was "deep throat", and in his memoir which was published in 1979, Felt denied to his co-author, de Toledano, that he was deep throat.
The fact that Felt denied for so long that he was deep throat, is somewhat perplexing, until you recognize that his admission to being the FBI leak, probably came about not from a deep desire to "come clean", but probably from the need for his family to monetize his notoriety at the end of his life so as to gain money from it. There is little doubt that Felt himself believed that his own actions taken during Watergate while with the FBI were not honorable, or he would have willingly and gladly come out as the source, upon Nixon's resignation in 1974, especially considering that Felt resigned from the FBI in June of 1973.
While the actions of Felt may be considered to be heroic by some, perhaps more because he was an important instrument in the taking down of a President that had abused his power and authority, Felt is no hero. Felt isn't a hero, because his actions were probably taken not for any noble reasons, but more out of spite for being denied the directorship of the FBI, as well as the knowledge that his future time was now going to be limited within the bureau, a job that he had loved and took great pride in.
While we may consider Mark Felt to be a great and important whistleblower, here too, he falls short, because unlike other whistleblowers that have identified themselves, and either faced the music, their possible jail time, notoriety, hostility, and legal problems, they have done so, because they stand behind their actions. Mark Felt, on the other hand, did not.