If your dog breaks its chain and attacks and kills a neighborhood boy, who may or may not have been taunting it, you would be held responsible.
You may claim a defense of: “This chain is strong enough as long as nobody makes him mad.”
But you would lose your case.
First, if that happened, the least of my concerns would be the lawsuit. I would be overwhelmed with guilt.
But I still think you miss my point. I carefully described a scene far different from some "neighborhood boy" casually walking by and having the dog suddenly snap its chain and attack. The provocation makes a difference. Who among us can say what our dog would do if someone started torturing it, especially if he feared they were trying to invade his territory?