Guilt by Association

When one thinks that someone else who woulds a belief that is done in bad faith or irrationally, it means that you also hold that belief in bad faith or irrationally.

There are many examples of what this could be. One of the most common is that someone else who held this opinion is shown to be a Chinese national sowing discord in America or a bot.

Another is the more mundane straw-man. Person A believes X and person B also believes X. However, person C shows person A is irrationally holding that belief. The logical fallacy is that person B most likely has different reasons for holding the belief.

It is vicious because it discounts someone else's views without engaging with them. Someone else can easily go and find a handful of beliefs that you have and apply this same thing by finding the worst or weakest arguments for your beliefs.