Hoover thought leftists working for the Communists wanted to infiltrate the schools and colleges, and United States politics. He thought they wanted to hijack the civil rights movement, and go after traditional religions. Looks like he was right.

Hoover being gay was a rumor started by his enemies to destroy his reputation. There was never proof of it. He did not concentrate so heavily on the mafia for a few reason, one being the FBI was more focused on interstate robbers, and the Narcotics Bureau and Treasury were going after them. Look what Hoover did when he did go after them. All out war, including using Cointelpro on them.