it's a sentiment that can actually be true.
Well, my problem with it is that it has "Often times" in it: "
Often times, the loudest and staunchest oppositions are the greatest offenders."
Sure there are very few who fit the sentiment like in your example, but it is not
often times. if it doesn't happen often times, then the sentiment is false.
You provided an example, but concluding this statement is true from one example is just Anecdotal Fallacy. You can't just take this single data point, this single person, and conclude about the whole population from it.
And, the sentiment is just a very disingenuous Ad Hominem attack meant to silence the opponent by presenting them in a bad way and weaken their standing, rather than address the points.
It also very unfair to people who are actually honest in their sentiments as to marginalizes them and render them powerless to fight for their belief by being accused of being hypocritical offenders themselves.