This is going to be pretty freaking long-winded, so bear with me.
When I first realized that there were 5 Darkin in total (based upon what Rhaast said), I knew that there was obviously a Four Horsemen + Conquest thing going on: Aatrox for War, Rhaast for Death, and the Darkin Bow for Blight - or in other words, Pestilence. However, something bugged me - Aatrox was the opposite of most depictions of the Horseman of War - instead of being a raging, wrathful warrior, he's completely calm and collected at all times; I don't even think the guy has a single line where he raises his voice.
Rhaast, meanwhile, is the opposite of most modern depictions of the Horseman of Death (potentially the Grim Reaper, if we're following that assumption as well), because whereas the Angel of Death is typically regarded as impersonal, cold, distant, and simply doing its job in regards to the Apocalypse, Rhaast revels in bloodshed, despises humanity, and enjoys every bit of what he does. He says it best himself:
('pologies Kayn) "This is the part I like best, the anticipation. No, it's the killing."
So, then, what exactly would this mean for the Darkin Bow? We can gather from what we've seen in Aatrox and Rhaast that they would likely be the opposite of modern depictions of their associated Horseman - but what is the Horseman of Pestilence seen as? Forgive me if I dip my toes a bit into themes associated with Warhammer 40k, but I personally would have to say that the traits associated with Pestilence would be thus:
Inevitability. Hopelessness. Impersonal. Giving up and letting go.
If we take this position, then we can reasonably assume that the Darkin Bow - or Pallas, as I shall be referring to it from now on - would be the opposite of those traits: Stubborn; Persistent; Hateful; Vengeful. This explains perfectly why it would allow Varus to it: it hates humans (possibly even more than Rhaast does?) and has some sort of reason for letting Varus be in control in the first place: Pallas wants Varus to get his revenge. This also explains why Varus is living on borrowed time, despite being in full control of the Darkin Bow: it's not Pallas fighting back against him, it's the aspect of the bow taking its toll on his body.
Finally, this brings us to my final train of thought: why would Pallas want Varus to be in control? The answer is simple - it sympathizes with Varus.
I know, I know, that's a stretch - but let me explain. See, the odd thing about Pestilence is that despite being the most blatantly antagonistic out of all of the aspects of the Four Horsemen (As Death can be seen as part of the natural cycle, War can be seen as righteous war or even defending oneself, and even Famine can be seen as fasting or penitence on a small scale), it's also the most impersonal: Death is, most often, caused by other human beings, or even our own stupid choices. War, obviously, is fundamentally personal - it's waging massive battles against another people or country because you were wronged or your beliefs tell you to fight them (or at least the aspect of War is - not so much in the modern world). Even Famine, which is brought about by the withering of crops and spoiling of food, can be mitigated if humans at least share what they have with each other - terrible famines occur when people refuse to do just this. But Pestilence is different: it's caused by microscopic organisms that can't even think for themselves, and while in modern times we have cures for most diseases, Runeterra probably would not (and that's assuming the diseases brought about by Pallas are even curable to begin with).
So what, exactly, does this mean? We know that Pestilence is the most impersonal yet antagonistic of the Horsemen, due to its associated aspect. This, in turn, would mean Pallas would have to be the opposite: the most personal and least antagonistic of the Darkin. So...why the fixation on revenge?
Because I believe that Pallas was, initially, the most benevolent out of all the Darkin: as the god Apollo taught the art of healing to the Greeks, so too might have Pallas done such for the people of Valoran, maybe even all of Runeterra. But, eventually, as humans saw the threats posed by the Darkin, they decided to do away with them as a whole - including Pallas. This would explain why Pallas is so stuck on vengeance compared to Aatrox or Rhaast: he cared about humanity at one point...and for his efforts, he got himself stuck in a bow (which itself was stuck in a pit). So when he calls to Varus (perhaps initially planning to take over his body, as Aatrox did and Rhaast hopes to do with Kayn) and sees his desire to have vengeance against Noxus at any cost, he obliges him...and after all, when Varus inevitably dies, Pallas gets his body anyways. It's a win-win.
HOKAY I'M DONE [sg-zephyr]
is a Darkin, yet Varus doesn't really have much of a lore in terms of who he was before the accident and after wards what happened when he became one of the Darkin. Heck he doesn't even have any special lines when in the presence of an another Darkin 
