Beautiful bird. I don't experience much aggression with NN roos... there have been a few though... and usually they got the axe.
Part of the problem is it has to do with lines also with NN mixed with ?? As far as I can remember, the hatchery stock have been all right.. but some did have the problem of breeding the hens too much or not being very tolerant of other roosters.
It is definitely genetic- jumper roos WILL throw at least some jumpers. If there is something about the line you like or need, raise surplus cockerels and cull the ones that jump.
I like to cull for attitude in my flocks. If a roo jumps, he is killed asap. I do not care for any reasons for his jumping.
Then I like to catch the birds and wave my hand in front of their faces... the ones that bite, especially the ones that bite and hold on with full force or bite repeatedly are also culled asap.
I also cull hens that do tasmanian devils while on the nest.