[Immerito]

“let these animals keep fighting until they killed each other”

Most dogs do NOT fight to the death; they have long inbred instincts that keep them from stopping short of killing another dog.

The fact that those mastiffs had not killed the smaller stray in the time from the beginning of the fight to the arrival of the police highlights that these dogs were not looking to kill the stray, just to establish dominance and make it explicitly clear that the stray was not to bother them again.

[kara37]

True. I show and breed mastiffs. They are not aggressive dogs. In fact, they are known for their good temperaments.
They may sound like they are ripping something apart, but they usually don't bite down to kill.
I have 5 mastiffs and they got a hold of a peacock the other week. I ran outside and found all five attacking this poor bird. I got right in the middle of the frenzy and grabbed the peacock. The peacock not only survived, he had nothing wrong except a lot of missing feathers.

The darn bird still won't leave my yard. He just pays more attention when my dogs are out.

[Venturer]

Now see that’s the difference between you and I.

I know absolutely nothing about dog pschiatry.