You cannot breed the aggression out.
Let’s think about guide dogs. There are hundreds of a scientists whose sole job is to monitor and select the right genetics. Breed the best of the best and raise them under very strict socialisation and training protocols.
At the end, you should have solid guide dogs, an excellent pairing of genetics and correct raising (nature and nurture).
And yet less than 40% of puppies graduate to guide dogs.
Because in 60% of cases their inherent traits (gundogs- tendency to chase, easily distracted, impulse control etc) show through. Sometimes as puppies, sometimes as they mature.
Nothing has happened to make this behaviour change. The instinct was there all along, suppressed. And 60% of the dogs will fail because of this instinct.
Now if this happens in probably the strictest and most rigorous selective breeding program in the world, overseen by top scientists, what on Earth makes you think Paul and Sue from down the road have managed to somehow 100% breed out the inherent, instinctive ‘Attack’ temperament that bullbreeds are wired for in just a few generations?
The pitbull that make a nice family pet and lives a long happy life is genetically an anomaly. And breeding an anomaly with another anomaly (“both parents have lovely temperaments”) does not guarantee an anomaly.