The journalist told ABC's "Good Morning America" Friday he thinks NPR CEO Vivian Schiller attacked him because she had a weak argument to justify his firing.
Williams was discharged by NPR after saying Monday he gets nervous at airports when he sees people in Muslim dress preparing to board his plane.
Schiller said yesterday controversial opinions should not come from NPR reporters or news analysts.
She also said that whatever feelings Williams has about Muslims should be between him and "his psychiatrist or his publicist - take your pick." She later apologized for that comment on NPR's website.
Williams said he was taking Schiller's comments personally.
"I don't understand why she has to get that low," Williams said on ABC's "Good Morning America."
"If she has an argument to make that I somehow violated some journalistic ethics or values of NPR, make the case. I think it's a very weak case. And so ultimately I think what she had to do then was to make it an ad hominem or personal attack ... that I'm somehow unstable or unrational."
Touting his long affiliation with NPR, including his efforts raising money for the public radio organization, Williams said, "If I was such an erratic character I think this would have come to the fore long ago.
"By the way, I don't have a psychiatrist!" he added.
He denied that his admitting to an emotion meant he was advocating discriminatory behavior, such as extra security screenings for Muslims. "I have a moment of fear, and it is visceral," he said. "Now, all of a sudden, I'm a bigot?"
Williams disputed NPR's excuse that his comments violated the organization's standards of practice, or that his feelings would in any way intrude upon his journalistic conduct. "I'm able to discern exactly what a feeling is as opposed to what is a public position, an ability to report and to hear what others are saying."
He also denied that his comments on Bill O'Reilly's show were different from comments he might have made on NPR, or that he was "catering" to a Fox audience.
"What do you think the issue is here?" asked host George Stephanopoulos. "Did the fact you were working for Fox become too much for NPR?"
"This is one of the things in my life that is just so shocking," Williams said. "I grew up basically on the left, I grew up here in New York City. And I always thought the right wing were the ones that were inflexible, intolerant.
"And now I'm coming to realize that the orthodoxy at NPR, if it's representing the left, it's just unbelievable. Especially for me as a black man to say something that is out of the box, they find it very difficult.
"And I think you're right, George: I think they were looking for a reason to get rid of me, they were uncomfortable that I was talking to the likes of Bill O'Reilly or Sean Hannity."
"For the right wing I think I was often a foil for their large personalities, and on the left I think I was a point of an unusual and unpredictable view, that I could bring insight and interest to their audience.
"All of a sudden NPR - especially this last group of managers - became vindictive, and as you can see personal in terms of their antagonism towards me."
South Carolina Republican U.S. Sen. Jim DeMint plans to introduce legislation to end federal funding for NPR because of the firing.