But if that's the case Harry would have at least law break two or higher.
Only if he actually killed people at the party.
Look, my theory is just that, a theory, and under my theory Harry would not have been able to kill those people.
However, I do not find the idea that the laws just exist arbitrarily and randomly likely at all. Jim has put a
lot of thought into the series, and from what I understand of the way that Evil Hat came up with the books Jim had a lot of input. I doubt that the implementation of the Laws is particularly divergent from the way he envisions them working.
Alternatively, harm caused by magic is "fed back" to the caster via the sympathetic link between caster and target, so even if the caster is unaware of the effects of his or her actions the metaphysical repercussions still find their way back. If the victim of the action is too alien (non-human) there is no real resonance, and no lasting effect to the caster.
In the core book in the lawbreaker section it says that you gain lawbreaker when you choose to break one of the laws
The "choose" might refer to the player. I'd have to read the section to be sure. I'll check WoJ over the weekend, but I'm pretty sure he was clear that causing a death with magic makes one a Lawbreaker, intent doesn't enter into it.