People get a little too hung up on the wording of "for enemies", it's dramatic but a lot of the magic stuff is dramatic so it pretty much goes with the norm. You could easily assume the HBP labeled it that just to get the point of it being a combat spell across in the middle of all the other random notes he wrote down, Harry clearly thinks it's not that out of place in the original canon, especially since there's no further instructions written down and it's been established in 4-5 that the really serious business curses need both power and immense intent to cast correctly.