I fell hard down the rabbit hole. If you look back at the breeding lineage of what are considered "top blood lines", you can see that the fault lies squarely with the breeders-excessive inbreeding-Kimbo, who is probably the most often referred to, was the result of a same litter brother-sister mating; One set of grandparents
Both grandparents have the same grandfather-one great grandfather. The COI of Kimbo has been variously calculated between 30 and 52%, depending which source you are reading. Acceptable COI is <20%.
Several of Kimbo's offspring, particularly when bred with one particular bitch, were reported to suddenly, without warning, launch into human aggression. At least 1 child died and several dogs were pts. Despite this, the breeders repeated the mating.
Blood lines from Kimbo are still being bred from. The breeder has said that owners are causing the aggression by injecting their dogs with steroids.(!)
There is no doubt about it that visually, XLs are impressive, majestic looking creatures; but when a high percentage of XLs in the UK descend from a heavily inbred line known for sudden aggressive traits, these incidents will keep on happening.
If anyone is interested, I read that a similar defect was bred into red cocker spaniels by breeding programmes focused only on developing colour, but many had highly aggressive traits (I think in 80s). Of course cocker spaniels are significantly less deadly.