My favourite thing about these guys as villains is that they genuinely care about each other and the Doom Tree. So when they come to Earth seeking energy, it’s not necessarily for evil gains like with the Dark Kingdom (steal energy, take over the world, destroy the universe, yada yada), it’s out of a need to survive, and if that’s at the expense of another species, then so be it.
It’s a really interesting concept, because although Sailor Moon is all about love and justice and saving everybody, if the roles were reversed, would she be willing to do what these two do in order to save the world she loves?