Apparently the bloodline is the only way to stop him. Yet every time he whacked the father (many times) he let the son live, even if he was standing right there.
So far as I know he doesnt have any morals or rules like Jason or Micheal, in fact he likes killing anything and everything because he believes thats what humans are for.
So why the spare the sons who will eventually ending up ruining him?
Maybe he wasn't sparing them and they were just lucky, like Jack with his mother figuring out the box in the nick of time.
shareIn the first two movies he had pretty specific rules. He spared several people who opened the box accidentally because his job was just to bring pain to those who were looking for it.
Children aren't generally interested in the extreme pain/pleasure stuff that Pinhead deals out, so there is no reason for him to pay attention to them at all.
Pinhead in 1-2 has a sort of innocence limitation in what he does. He spares those who never touched the box, and in Kristie and the mute girls cases he spared them simply because they either didn't actually open the box or was coerced. He wants to bring experience to those who seek it.
In Part 3, he kinda blows all the rules and goes apesh!t on the city. Let's ignore this outlier.
Part 4 he seems to be following the rules of hell again, in an off and on manner. The two cops were essentially innocent, but since it was a scene post HR3 he transformed them.
Hellseeker shows him making a binding deal and taking an innocent as well as several others in exchange for Kristie.
Inferno shows him only taunting a detective. The only true innocent killed was the metaphorical innocence of the officer in child form.
Deader he dispatched all of the Deaders... who violated the rules of hell itself. So I guess he still has some morals in that situation.
Hellworld should not really count because he was more or less a hallucination caused by a potent psychotropic drug.
While I'm on the subject of rules of villains. Jason just kills, and really his rules fade into obscurity as the films progress. Myers tends to try to kill family members, and pretty much anybody who gets in his way of doing that. ( In the case of Ressurection... anybody messing with his stuff)