Why Darkness?
Should a 1|1 and a 7|1 have equal chances of leaving or should they be ordered by hp and then by attack?
Equal chances.
The mechanic fits Darkness. Many of Darkness's cards rely on taking advantage of an opportunity (e.g. Steal, Voodoo Doll, Nightmare). This card takes advantage of low HP and (to a softer extent) low number of creatures. Rather than completely destroying the creatures, it sends them away for a while.
The theme fits Darkness. When you think of a member of the community leaving, you may feel sad or hurt. Leaving is always associated with pain and unfairness, like stealing (permanents, quanta, or HP) is.
The theme and mechanic doesn't have a connection to another element.
It's not Entropy because the targeting isn't random; there's a method to the madness. In addition, the second effect isn't entropic at all. As for the theme of leaving, there's no way you can fit that into Entropy.
It's not Time because Time has nothing to do with low HP, and this spell doesn't deny draws like Reverse Time does. Since your deck is superficially unchanged, it's more of an insta-kill than one of Time's draw-manipulating mechanics.