You have to ask yourself how much of a mean stud’s temperament can be attributed to nurture vs. nature.
Sure, aggression can be a heritable trait. But look at how most studs for racing are housed: solitary confinement, perhaps staring at your competition all day, no direct contact with other horses but for the act of mating itself and even then those pesky humans still get in the way…I don’t care how much the brass is polished, living conditions like that can make for one helluva son-bitch.
You also have to keep in mind that a certain amount of natural stallion behavior --biting, rearing, just being rank in general-- stems directly from testosterone. Their daughters and gelded sons just won’t have the same 'roid rage fueling them.
To add: I had the honor of meeting Dynaformer in person this past May; though the chain-shank sewn into his halter gave evidence of his difficulty in being handled, not once did that stallion so much as flick an ear back the entire time he was being gawked at. He stood there looking pleased as punch with himself, waiting, just waiting, for someone to make the mistake of offering him a finger-food snack I suppose. Given his reputation I was expecting a full ears pinned flat back, mouth gaping open with teeth bared, rearing and striking display upon being approached by leering tourists.
I never met Barbaro, though I must say that he did display a certain kind of aggression on the race track, a trait that is most desirable in this industry: the attitude to win at all cost.