But does the Watchers Council have any legal jurisdiction, I don't think so.
When a slayer got out of hand they transported her, but they hid her in the hold of a ship they didn't transport her in the open, on a plane. They had drugs to make her weak, as used on her 18th birthday.
In checkpoint it was said they could have a person kicked out of a country by messing about with bits of paper, not by legal authority.
That sort of thing indicates they are a self professed authority and not a legal authority.
but they could have told Buffy to kill but not according to law, so Buffy could still be charged with murder.
With regard to the witches, those witches are part of society and are subject to society's laws.
The kids (demon) were there for the blame to be put on witches. The demon promoted the people to get the witches. The demon was outed but who killed the Demon?
Because society's law doesn't accept demons, it might classify the demon as a seriously bad person (or similar) and as such the person who killed the demon would be charged with that murder. And the Watchers Council would not be recognised as a legal authority.