This really does not concern me. I am only concerned that he is not paying abeisance to any demon, and he's sincere in his outlook. His ultimate afterlife fate, some millennia in the future, in a theological setup that I grudgingly accept as appropriate for the setting, but I give no sympathy whatsoever outside of this game, leaves me utterly unfazed. It's no chance that I only play irreligious, secular, or pagan mages.
As for my character, as I said, he sees "advanced Chthonic magic" as an unpleasant but morally neutral tool. The dark forces he invokes for that, are not benevolent nor sympathetic to humans, the way a blizzard or the unrelenting desert sun are not, so it stands to reason that invoking them needs malevolent means, by sympathy. They are "wicked" the way the knives of a surgeon cutting up a patient is.
I would point out that there are ways of invoking Chthonic power, that while "wicked or malevelent" are not really "evil", such as invoking dark gods or spirits, cursing someone who was your enemy anyway, animal sacrifices, offering the lives of one's enemies in combat. etc. That ethical limit satisfies my PC, and his player, neither of whom really places any faith or sees any redeeming value in Middle Age Christian theology.
The judgement, while made "right" by overwhelming might, needs not be acknowledged as just. One may well be see the cosmos as unjust and tyrannical, and prefer to be true to one's conscience and free will, whatever the price. Insert quotations by Aeschilus' Prometheus, Milton's Satan, etc.
That is the only viewpoint that matters, IC and OOC, as long as I'm concerned.
For what it may matter (and it does, since I'm our group's assistant/counselor ST, in charge for rule system advice), that theory is utter crap IMO. Mages who undergo any kind of immortality transformation do not lose their souls nor really die, souls stick with the mind until consciousness is not snuffed out, and body & spirit only undergo a radical transformation. I'm a fervent believer in the theory that identity moves with consciousness, and souls (if such a thing ever exists) follow, and such a model applies to transformation of the body in ArM metaphysics. Transforming oneself in a faerie or magical creature does not "kill" you anymore than taking the shape of a wolf with a spell or power.
Also, while my character is a Merinita, he's not necessarily going to undergo the Becoming as immortality method of choice. He has some insights in the theurgic Philosophers of Rome too (even if that part of the background has not seen any real development in recent years of play), we have had some significant brushes with an alchemical-necromantic mystery cult which holds the secrets of the Philosopher's Stone (gold is flooding Northern Europe!) and the Great Elixir (which may just be an infernally-corrupted Norse offshot of the Green Cockerel), and we personally know an Archmage who was made immortal by tasting the fruits of the Tree of Life, and we went to the Garden of Eden and of the Hesperides ourselves.
Therefore, I do not lack the background justification to set my PC on whatever immortality path I fancy, be it Becoming or magical/alchemical immortality. I might just do the latter and pursue the Great Elixir, or choose incomplete Becoming, since the character is a thoroughout spontaneous magic specialist with serious limits to use formulaic magic effectively, and full Becoming is incompatible with that. Becoming an immortal is an important goal for my charcter and others in the group (personally knowing an immortal NPC tends to trigger such feelings), but it's a long-term one. With Unaging and Strong Faerie Blood, he has plenty of stopgap anti-aging measures.