He acted out of selfish malice. He deserves neither hero nor vigilante. Backstabber might be more descriptive.
Felt acted out of duty to his country. He is a true patriot. He obviously felt an obligation to the truth, he felt the constitution was being undermined, he felt the FBI was being misused by Hoover's successor. He uncovered corruption at the highest level of our government and brought it to light for the people.