Saying someone is an idiot is an opinion, there is no standard for this claim to be proven true or false.
Saying that someone is a murderer is not an opinion, regardless of how you came to that conclusion, you are making a statement of fact.
The US justice model is seriously janky if you can accuse random people of being murderers, ruin their lives, and get away with it.
My opinion: whether your 'proof' comes from god, the tarot or a dream, if you can't provide verifiable evidence of your claim for a crime committed in the physical world, you either need to shut up or agree to face consequences, whether you really believe in your source or are just a liar should be irrelevant. That right should certainly not be more important than the lives you're ruining.
The US's standard allows for the eventuality of the Crank in the Public Square: it's incumbent upon the general public to understand that the Crank has no particularly good reason to believe the things they believe (and say), and thus aren't worth listening to.
Overall, this results in (arguably) more desirable outcomes: we don't subject Cranks and the legal system to unnecessary interactions with each other, because what the Crank says simply does not matter.
(The equation changes when the Crank does begin to convince other Cranks to hurt or harass their subject. But that's a matter for incitement or other standards, not defamation.)
Saying that someone is a murderer is not an opinion, regardless of how you came to that conclusion, you are making a statement of fact.
The US justice model is seriously janky if you can accuse random people of being murderers, ruin their lives, and get away with it.
My opinion: whether your 'proof' comes from god, the tarot or a dream, if you can't provide verifiable evidence of your claim for a crime committed in the physical world, you either need to shut up or agree to face consequences, whether you really believe in your source or are just a liar should be irrelevant. That right should certainly not be more important than the lives you're ruining.