I think it's less likely that the Mara are shed karma, or elemental divinities that chose not to join the Arcana. @AdamKoebel has mentioned a few time that the Old Gods are not anthropomorphized (and the same goes for the Arcana, too) in the way that we as viewers tend to think of them as. Those that chose not to join the Arcana became more absorbed in their elements (to wit, Agni mentioning that when she returns to the elemental plane of fire she would not remember the things she remembers as an individual in Utrix's Sanctum).
The Arcana are depersonalized, but have an identity, and their purpose is to cleanse souls of their Dharma, reincarnating them according to their Karma, until they "ascend" to enlightenment and unite as Being with the Fountain, away from Becoming of the material world.
What is "Becoming"? Dharma. The Mara strive to upset the work of Heaven and prevent the wheel from turning; they seek to preserve Becoming and never to Be. The things that they offer are material wealth, power, etc., and in exchange they pluck the hapless, villainous souls from the wheel - they peddle Becoming to preserve Becoming.
For the Mara not to have existed before the great cycle of reincarnation means that they are tied to it, they are an offshoot of it. When there is a force that calls for all to be joined in Being, the Mara arise as the counterforce, the gravity of Becoming.
The Mara are the shed Dharma of the world, becoming stronger and more potent the closer the world of Becoming is to the undifferentiated Being. They are the activity of the world opposed to the static nature of the Fountain.
In this way, they mirror the tower by sowing destruction, but they are its opposite in that they lead not to the star, but an inversely static Becoming that is the opposite of Being. They are irascible and incomprehensible in truth because their goal, if achieved, would result in their own undoing and a uniformity that, through its antipody with Being, would paradoxically lead just as certainly to the unity of Becoming. Thus, they are ever striving, perhaps even ever winning, but they will never win.