FWIW, "You have to want someone dead, not necessarily the same person" is essentially how "intent to kill" works in a legal sense. That is, the distinction between murder and manslaughter is whether, by your actions, you intended to kill someone; under the law, it doesn't matter whether the person actually killed was the person you intended to kill, or not.

Not that the rules of magic are necessarily modeled after modern US jurisprudence, but it might be that they both reflect a deeper moral concept.

Wanting to kill a specific person may be a requirement for fueling the spell, sure, but I don't see why that necessarily entails everyone else being immune to what is essentially a profoundly lethal effect. Once a bullet is in the air, it doesn't matter what motivated the firing of the gun.

The bit about nobody mentioning collateral damage sounds like an argument from silence. I'll tentatively grant you the point about "no possible defense", but to me it seems like Moody could well have been talking about deliberate, cold-blooded murder rather than all possible circumstances. I mean, by the time of the "no possible defense" line he's already name-dropped the Monroe Act, which is nothing if not a big, fat exception.

"if you're arrested for killing with it, there's no possible defense", and "I meant to kill the Death Eater, but I hit the bystander" is a possible defense.

Well if you were using it fighting Death Eaters under the Monroe Act, and accidentally killed a bystander, and had fellow aurors to back up your story, you probably wouldn't be arrested.