They're pits crossed with molossers - both aggressive types of dogs the mastiff side included. Mastiffs are usually placid until they're not. There are some with screws loose especially now everyone and their brother wants to do bite work with big scary looking breeds and they breed their own "working lines" of big mastiffs which are more likely to go into an xl than a dopey english mastiff that trips over its own skin. Then those lines end up in the general mastiff population and who knows when it will rear its head further down the genetic line.
Pits are self explanatory aggressive, there's a common misconception that the human aggressive dogs were culled - there are several famous dog fighters who kept on human aggressive dogs because of their performance and bred from them, and then less scrupulous (ha) dog fighters who wouldn't cull for human aggression at all.
Plenty of dogs have a higher inbreeding rate and they aren't going around killing people, Kimbo's COI was 31% which isn't staggeringly high compared to breeds who are perfectly affable - https://www.instituteofcaninebiology.org/blog/inbreeding-of-purebred-dogs-determined-from-dna
As to your last point, chavs owning them doesn't mean they'd behave for a naice middle class family. In the US where the pitbull problem is out of control there are plenty of nice normal families pulling their hair out over a pit (which is what an XL is) they got from a rescue wanting to eat their grandkids or the neighbour on a bike or scaling 8ft walls to kill neighbourhood dogs even when they've exhausted professional trainers etc.