Aatrox was the first of a species they hadn't written into the story yet. It wasn't that they wanted us not to know who the Darkin were, it's simply that they hadn't actually written that yet. If they had any firm ideas, they were scrapped. So he was released with the necessary lack-of-character in his character to fit that.
But now, they're giving him and the Darkin solid connections to the Targonians, Gods, and the Void. With their place in the universe set firm, he can be written as the crazed, trapped, revenge-bent slaughterer of gods and men that he always was intended to be. His new voice actually fits the theme of a man/demon who can swing once and take out a platoon and enjoys doing it. His old one didn't even in the slightest-- particularly in regards to his relation to Tryndamere. The old voice would be better suited to the kind of mage who controls the emotions and minds of his enemies from the backline or off the battlefield entirely, not anyone who carries a sword and dives into the thick of things.
Not sure why there are suddenly so many people who supposedly _liked_ not being given any insight into the character's personality or story via the VO, when his play rate was sitting near the bottom for years. Seems to me they just like complaining about change.