I will say I am actually okay with keeping most of varus´ original lore and changing him into a darkin. The problem I have is how his story is way less tragic now, with the val and kai sideplot stapled on top of it. The whole darkin lore, in general, doesn´t really offer much for me besides "muh destroy the world for reasons" and it weakens his character motivation. In the old lore he commits to punish the people responsible for his situation including himself.
There is also a touch of desperation intertwined with his actions. Varus the man who has nothing to lose because he has already lost everything and it was his fault. In the new lore as far as I understood it he is just along for the ride after he lost his family. Getting ascended seems so disconnected from what he did before compared to his old lore when it was the sole reason he relinquished his former self to obtain that power.
And then he gets corrupted by vengeance somehow and becomes a warmongering lunatic because we have to get our darkin somehow and since darkins are written as flat as a piece of paper his character development or and let´s say my investment with that character takes a nosedive. What was great about the old varus was a short story about how a good man became a monster, a menacing force of evil and the reader could see every point in that transition and understand the decision that led to it.
New Varus´ on the other hand gets replaced at the end by a completely different character which makes perfect sense considering they had to shoehorn the stupid darkin lore in somehow. Now he is just evil because we have this ascension mambo jambo and some wars happened and now everyone can get corrupted. That is the equivalent of corruption we see in Star Wars and it´s not good because a turn from one side to the other is way more satisfying when it´s character driven rather than having an anonymous dark force which does it seemingly arbitrarily.
edit: I forgot to mention how I loved that old varus could achieve neither one of his two goals. He failed to protect his family first, then he gave up and the evil was freed regardless. This was way more dramatic than what we have now.