Not self-defense. Pitbull was not attacking man.
Pitbull attacks my dog and it will be a dead pitbull.
Another mans property, was destroying his property. So he used a tool (knife) to stop the destructive act being performed on his own property.
Self defense need not be demonstrated.
My dogs are part of me.
If anything attacks them, it attacks me as well.
And I will respond harshly.
And I will be backed up by police as has happened before.