Is it possible that the corruption that changed Varus turned him into a Darkin?
One of the lines from the Vastayan stories made the implication that Aatrox is actually the blade and not the being holding it. Aatrox now being an embodiment of war and bloodshed.
This made me wonder if that corruption beneath the Ionian temple wasn't Darkin in origin. In being embraced by it, Varus became one too, his soul bound to his bow rather than his body, and his single-minded pursuit of vengeance becoming the sole definition of his being.
Also, looking at Varus's splash and how his eyes glow purely in the same purple coloration as the energy around his hands and bow, like how Aatrox's eyes and sword glow the same color. It's most noticeable on Justicar Aatrox's splash art, where his sword hand is glowing in almost the same way that Varus's left hand is on his base splash.
Maybe I'm just thinking about this too hard. I don't think the 'corruption' was brought up in any of the vastaya stories, but I'd have to read them again. Varus definitely comes off as more demonic than just bestial to me.