I would even go further and assert that the one denigrating a person with whom they disagree is who truly is demonstrating bitterness as well as anger. They are showing an inability to be confident in what they believe and so they must impugn the one they are in conflict with to settle their own doubts about their faith. I'm not a psychologist, but I recognize "projection" when I see it.
That is true indeed.
But it's so much easier to dismiss a former Catholic's position if you can paint it as being the result of emotional failure. It's an attempt to kill two birds with one stone. It attempts to discredit the message and the messenger.