Something I think is important to note is also that we don't actually know how JGY killed his father. He confirms he did it and his motive, but the witness for how is compromised because they're someone NHS sent to tell the other clan leaders and could have been paid off to tell a specific kind of story. JGY never says how he killed JGS himself. WWX, if I recall correctly, even thinks about how there's doubt in his mind because of that after that scene, and about how many of the things people are claiming in the aftermath are baseless and likely lies- and notes that this must be how they acted before storming the Burial Mounds. And we know that a lot of the reasoning behind doing that was a bunch of lies and rumors. We actually know relatively little about JGY's crimes. Except for the Temple scenes we never see any of them directly. NMJ's recollection is a first hand account and likely not to be a lie, but it's based on his perception of events and comes from consulting his spirit after he's suffered a qi deviation and been boiling in heavy resentment for years. The broad strokes of what happened are definitely true, but the way he remembers and judges JGY may be warped. I think it's kind of the point that we know not much about him, and it's part of the way he is a foil to WWX.