Oh, OP.
I don't know. I may be a shit parent. DS1 has been both bullied and a bully. His behaviour is often atrocious. He has various diagnoses, which sometimes provide an explanation, but never an excuse.
The times I have come down hardest on him have been when he has mistreated others. His 12th birthday was cancelled (party, family coming up, big presents) because he and a friend had engaged in a campaign of behaviour against another boy they disliked which culminated in them stealing his bike and dumping it. I actually got reprimanded by some of the services which support him for the scale of the penalty we applied to him for that: they thought I was OTT, I said I could let an awful lot go but not that sort of cruelty to another child.
I'm sorry for everything DS1 does that hurts others and I promise I try my very best to manage his behaviour and I don't let it go when he engages in bullying. The same as I try my very best to protect him when he is the victim and I don't let it go when he has been bullied- and things have been awful at times, last year I had to face down a gang of older teenagers who came to our house for him.
I was bullied myself. I will always, always react if ds1 bullies others, I wish I could ensure he never does and I am sorry some parents won't accept that their children do terrible things.