The pure lines were bread for gameness, so when they fought they could become badly injured yet would still carry on the fight. This gameness also means they are a high energy breed, that energy is ideal for their intended purpose, you don't want the animal to stop until the job is done and they won't, that is the instinct of the brain and it is genetic to the animal.

A few more selected breed features are a large strong jaw which wouldn't snap or break easily and high muscle mass to protect the bones if trampled. small stature, small legs which are less easily damaged.

The features above are ideal for the intended purpose of the breed.

The issue is not the breed, the issue is the owner. The owner projects humanity onto the animal and sees the gaping jaw as a smile and the high energy as playfulness. My uncle has several as guard dogs, they make good guard dogs but even properly trained the prey instinct exists so they can cause problems.