To me, the greyness of characters is fine as overly righteous character can get stale but Riot is using it too much. Riot is telling the stories like all characters are grey besides a few horrors who can't be anything but evil. Changing characters who are seemingly good before to grey to give them more character when sometimes what stories need is good people.
One story type of villain I like in fiction is the unstoppable villain. As in they aren't stopped by morals or talking, they are there to do their things and nothing else. My favourite enemy of all time is The Fury, from Alan Moore's Captain Britain. This 'villain' isn't evil but it is by it's design as it's built on the idea of hate. Jim Jaspers hates superheroes, he blames them for the worlds problem and in his insanity creates an impossible being to rid his world of them, programming it with one task, to find a target and kill the target. This concept to me is better than most other villains in the MU as it's something everyone fears. Cap, Spider-man, the X-men, Thanos, Galactus, Doom, you name them and they will fear this creature because it can't be convinced, it can't be reasoned with and it has no morals.
This is my issue the Darkin change. They went from an outside unstoppable force that decimated Runeterra to a bunch of human made gods that warred with each other, slaughtering thousands in the process. We are made through the stories to sympathise with them as they 'had to fight the void', got PTSD and that they could be redeemed because they were once good.
If they had remained from the outside, beings from another world that were not going to be convinced, were not going to be morally switched and were there to do nothing but slaughter and conquer it would have made them far better as characters because there is no greyness about them. The idea of different races and nations of Runeterra joining together to fight a single opposing force that wasn't going to stop to me is far more compelling of a story than some infighting. Then when they are 'picked up' it would have also given them more purpose as well. Aatrox isn't a Ascended who whines over what he lost (because of his own doing) and is in pain of what he is now, he's a unstoppable being that had to be tricked into his own weapon, now alive taking over hosts to end Runeterran's existence.