The Darkin might be working for the good of mankind - by bringing out the absolute worst in them
So now that we might have had the second Darkin to make her appearance, there seems to be a common element: use death and tragedy to elevate a chosen group of people on a whole new level, mostly when they are fighting against something.
It is not exactly a secret that humanity as a whole is a nasty species. We are in a constant process of tearing each other apart, and the desire to put onself over others is an evolutionary trait so embedded in our existence we don't even question why the element competition is so central to our culture. We are mean, cunning, reckless and cruel. You will find no animal with these traits. Animals kill because they are hungry and fight because they are threatened. But cruelty and hartred are exclusive to a sentintent species.
And maybe that's also the reason we can deliberatly choose to not be like that, but that doesn't change the fact that mankind is the most productive when it is working against someone. Most ideologies work that way, usually uniting their believers against an external enemy: the flighty capitalists, the non-believers, the nazis, the communists, or whoever might fit. And when going against an enemy mankind has shown a potential one would have never thought it would be possible before. The big massacres of the last century, the technological development in terms of weapons... we as a species have enough nuclear firepower to destroy out entire planet several times over, and that's not even accoundingt for chemical and biological weapon-tech.
Now what if you could harness that potential somehow? If you could bring out all that rage, harted, greed and whatever else you have inside of people and make them let them empowered by it and embrace it? Create people most rotten and they shall be the champions of their species - at least that's the theory behind it.
And I think this is exactly where the Darkin start to take influence on the world. But not like Bard who only makes minor cryptic changes that might or might not have a relevant effect in the next 100 years - and who doesn't give zero shits about humanity as long as they don't mess around with the very fabric of the reality.
No, they go out there and spread their "evil" around mankind, in the hopes of finding promising individuals that can act as their champions. Aatrox did that with Tryndramere, his "greatest creation", an avatar of rage. Something similar might have happened to Vayne who is a harted-fuelled killing machine now. And I'm sure the remaining three darkin have also special aspects to them. One of the might be "Greed"/"Hunger" who I'd could bring into connection with Swain but we lack evidence for that yet. And who are the other two? We don't know...
As to why they are doing this? No clue. Maybe they just try to see what mankind if capable of? Maybe they were created by some crazy ass mage in acient days? Maybe they just form around every sentinent as some sort of universal force? Maybe they work against each other and look for specific champions to take each other out?
Hopefully Aatrox gets a VGU soon (because if you just do a class-update like with Mordekaiser... well. we all know how THAT turned out) and a ton of new lore with that...