White unicorn is opposed to chaos and should be hostile to all chaotics.
Conversely, black unicorn is opposed to order and should be hostile to all lawfuls.
Of course if you are L+ and want those 7 potions then it has to be either very difficult or very expensive (in terms of alignment drop) if not outright impossible to get them.
L+ means you are lawful from the beginning till the end, save for a few minor chaotic acts. Killing a greater white unicorn is a *major* chaotic act and should have major consequences.
Going further in this direction, Hotzy should be able to inform the player of the dead glade at level 30 and a greater black unicorn that is "a powerful being of darkness, that seeks to bestow power for furthering the coming of chaos."
Depending on your alignment, either white or black unicorn informs you of his opposite side's location.
For neutrals, both unicorns should be non-hostile. Alignment drop for attacking a non-hostile monster should be adjusted to be equal to a fraction of alignment gain for killing a powerful and very chaotic being, so the net gain would be significantly positive.
The same goes for killing the greater white unicorn - massive alignment drop for the great prize. Makes sense I think.