There is a sutta (I believe it’s in the MN?) but after a sincere effort, I can’t find it (nor by searching on the internet).
In it, someone asks the Buddha which bad kamma brings the worst results: bad physical, verbal, or mental kamma? And they guess that bad physical kamma must be the worst.
But the Buddha counters with an analogy, showing actually it’s mental kamma which has the potential for bringing the worst resultants:
If a man went to the crowded town square of a village, sword in hand, and said “I’ll reduce you all to one giant heap of flesh”. Would everyone laugh at him? Yes.
But if a powerful sage/ascetic/samana/shaman/holy man/recluse/whatever with psychic potency went to the same crowded town square of a village, and said “I’ll reduce you all to one giant heap of flesh”, then actually did so with one giant feat of psychic power, then that shows that his mental kamma yielded the greatest evil, and therefore, the biggest, baddest resultants will ensue.
The Buddha also mentions, as a means of proof, an obscure reference to this actually happening in the recent past (at that time). The listener in the Sutta acknowledges that yes, he heard of that event, and implicitly believes it was a true demonstration of psychic potency (which thereby supports the Buddha’s assertions with believable-to-the-listener proof).
Can anyone remember which Sutta this was?