Aggression is pretty clear cut.

1) Destroying or taking someone's property. This includes all forms of vandalism, theft, and fraud.
2) Destroying or coercing someone. This includes all form of murder, rape, kidnapping, and assault.

You don't even need a 2) if you believe in self-ownership. Private police forces will not initiate aggression except as a response to aggression. If they did, they wouldn't survive their competitors. Again and again I need to emphasize the fact that private entities are more accountable to liability than states are. The free market simply cannot support the kind of wholesale tyranny that states are famous for, try as anti-capitalist muckrakers may to misrepresent the issue.