Greyhounds are the most likely of any breed to kill a cat or smaller dog. I know I feel a lot more uncomfortable when I come across off-leash lurchers and greyhounds than I do bully breeds when I am out with DD's Chihuahuas.
If this was true we would have large numbers of greyhounds responsible for attacks on small children. Luckily, all dogs, even XL Bullies, know the difference between a small child and a prey animal.
I have no idea what is going in with XL Bullies. I agree there have been a disproportionate number of deaths attributed to them but I think we need more research before we can determine that there is something inherently wrong with the breed/type as a whole.
Atm, it appears the issue has more to do with bad ownership/the type of people these dogs attract than the breed itself. Almost all, if not 100% of the attacks, can be attributed to owner negligence and a lack of training and socialisation.
Banning XL Bullies or removing them from homes with children is not going to resolve the massive issue we have in this country with backyard breeding and inbreeding, puppy farming, and irresponsible ownership. Any large breed could become dangerous with the same set of conditions the XL Bully is currently in (over-breeding/inbreeding, irresponsible breeding (breeding for looks instead of health and good temperament) and poor ownership).