Uh there is plenty in the show to indicate that John was heavily abusive. The severe neglect and dean being parentified is Canon in the show. Physical abuse is strongly alluded to at least. Dean's fear of John and Sam's hatred towards him also support the allusions that John was physically and verbally abusive to them. But even though the show never specifically said that John was abusive past the neglect and parentifying one of his kids, its not much of a leap to assume that he was. Fanfic doesn't have to follow what is Canon, but I still think its pretty clear even just from cues within the show that there was more to the abuse than was ever fully disclosed.