Isn't there some speculation that it was a lurcher cross, anyway? If the family own bull breeds, it's not inconceivable that it was crossed with a bull breed. Likewise, she may have been caught in the crossfire of a dog fight between the lurcher cross and a bull breed.
Either way, I think that if you own a large, athletic dog capable of causing serious injury/ death then you must take that ownership responsibility extremely seriously and not make assumptions about its nature. My dd was bitten on the eye by a very small breed (a supposedly friendly family dog), she escaped with a small scar and a lifelong fear of dogs. Had she been bitten by a bull breed, at the very least she'd have lost the sight in one eye.
I say this as someone who grew up with a beautiful cross breed rescue dog with a wonderful temperament who never bit or showed aggression to anyone in her 17 years. My sister owned a Staffie who was similarly beautifully soft and daft around people. But around other dogs she was more temperamental, and dsis had to keep her wits about her.