And its that they were human once and as such their personalities are not immutable. They are not demons like Evelynn or Tahm that seem to have purely negative functions set in stone that they pursue endlessly. I've seen the argument that "no darkin would ever be repentant" for why x weapon or person couldn't be a darkin, but I think this is a little presumptuous. Just because Aatrox, Rhaast, and Varus are not repentant doesn't mean a darkin couldn't be. Being stuck in nothingness for as long as a darkin has you're bound to do some self reflection. Who is to say we couldn't get a repentant darkin? Or one sympathetic to their host?
WAAARGHbobo has said at least a couple times now that redemption for Aatrox is possible and true redemption doesn't come without some real self reflection and desire to change. Varus' negative emotions at times overwhelm his human hosts; who is to say that their positive emotions could not positively effect him in return eventually? And Rhaast and Kayn already bicker like an old married couple, who is to say that they could not warm up to each other some day? These are just some examples of what I mean, the darkin are people - people who have twisted their bodies, done some horrible HORRIBLE things, and been in magical solitary confinement/sensory deprivation for god knows how long - but people nonetheless. Their personalities are not set in stone, anything could happen. I don't foresee redemption as their endgames (except maaaaaybe Aatrox because Odin mentions that so much) but y'know, it's not as impossible as people seem to think.
Personally I'd like to see a darkin champ who IS repentant or a darkin champ who IS sympathetic to their host. I think that would be an interesting change of pace.
TL;DR The darkin have the capacity for any kind of personality since they were once human. Personality should not be a factor in debunking who is or isn't a darkin.