You are missing the entire point

> The message is that the others are bad

The message is that they're bad and the fact that they did these bad things proves they're bad.

And the key thing here is that we need to decide if we believe "they did these bad things". If the person reporting them is well known as someone the is truthful and trustworthy, we're likely to believe them with little proof. If the person reporting them is well known as a bad person that does things to harm others for their own benefit... we're less likely to believe them until we can verify the truth of their statements.

You're completely skipping over the "is this person telling the truth" part; I assume because they're saying things that fit in with your pre-existing view of the world. And that's not a good thing.