I certainly agree that it must indeed be rare if people like you and Molpet who have had a life time experience with chickens never saw it.Most of the responses from the more experienced keepers tend to state males attacking chics is rare, if it's happened at all for them.
Pure speculation from my part, but suppose there is link between an adult male's rotten behaviour with the hens and being aggressive with the chicks. Most chicken keepers don't keep a rooster that isn't good with the hens long enough to see how he behaves around chicks. Just like in nature, that rooster would maybe not have a lot of chance to become a father ?
My keeping circumstances did not change but luckily the next three males that hatched here were not like this. What changed was the flock's dynamic. The aggressive rooster Théo was taken away from his first flock when he was three months old and arrived here in a tight group of six two years old ex-batts who had never seen a rooster ; it was very hard for him to make his place and he was bullied for months. The other males were born in a flock where there was already an adult rooster however horrible Théo was to the first male chick.If on the other hand such attacking chick is more common than I've portrayed then I would be interested in knowing the keeping circumstances.
I believe one would find that an explanation for the behaviour could be found there.
One thing I do know is that Théo behaviour did not come from having aggressive male genes through his father.I think the problem is that because the behavior isn't activated in the hens it's assumed not to be there. But we know that females carry genes that they pass only to their sons. Although no studies have been done that I am aware of, certain aspects of behavior could very easily be that kind of maternal inheritance.
For one thing, the farmer who gave him to me keeps a multi rooster and multi generational flock and any rooster who showed this type of behaviour would be instantly dispatched.
And the other thing is that I have observed countless times many bantams from the same lineage who free range on our village road. They usually stay in couples or sometimes two roosters with two broodies, watching over armies of chicks. The roosters are extremely protective of both the chicks and the broodies.
If genetics was for something in Théo's behaviour it must have indeed come from his standard sized mother.
That is good news and I hope comforting you that you did what was right.Follow up:
The chicks are not hiding close to the broody hen any more, but exploring, investigating all areas of the run. I was pleased to see how active they were, despite a drizzling cold day. The older chicks will be two weeks old tomorrow.
This evening before dark I brought a small amount of shredded cheese to the run. The hens milled about, excited about an expected treat. What surprised me was the chicks joining in, dashing in between the hens to snatch a crumb here and there.
Total change in behavior 24 hours post-rooster.