Hostis humani generis (Latin for "enemy of mankind") is a legal term of art that originates from admiralty law. Before the adoption of public international law, maritime pirates and slavers were held to be beyond legal protection, and could be dealt with as seen fit by any nation, even if that nation had not been directly attacked. The term is also used in the present to describe the status of torturers.
A comparison can be made between this concept and the common law "writ of outlawry", which declared a person outside of the King's law, a literal out-law, and subject to the violence of anyone. The ancient Roman civil law concept of proscription, and the status of homo sacer conveyed by proscription may also be similar.
Read more about Hostis Humani Generis: Background, Theorized Extended Usages of The Term, Actual Extended Usages of The Term