The Reaper voidsent are definitely not good. Orcus was a Reaper's voidsent, and he might be the most absurdly evil thing we've ever fought. Not the most accomplished evil we've ever fought, granted (who gets that prize depends on your view of 'evil', but it probably either goes to Meteion or 'whatever Ascian enjoyed the Calamities the most'), but nobody can argue Orcus doesn't have the spirit. The most we have on the lore of the Avatar is the website lore saying that it's purely transactional--it eats the souls of things we kill, which is... actually really terrible now that we know how reincarnation works, that's permakilling to a level that not even the biggest villains of the game so far perform. I'm pretty sure they might actually be worse than most voidsent, which I think just go for aether.
I'm perfectly willing to agree that Voidsent could have some good in them, but... well, so far we haven't seen a shred of evidence for it, and they've had a hell of a lot of chances. And I wouldn't even see that as anything but a horrible tragedy with none of them at fault; after ten thousand years of a dog-eat-dog world fighting over scraps, where the only thing that's ever been rewarded is malice and cruelty, I'd hardly blame them for not having a shred of kindness left in them; even if that wasn't wiped out in the Flood of Darkness, it hasn't been rewarded in millennia. That isn't cruel or harsh; remember that we killed scores of Sin Eaters despite mourning who they were, because we knew that what was there is gone. There's no indication that voidsent are any different in that regard; I'd say that the difference is that some seem a bit more intelligent, but then I remembered Vauthry and Titania count as Sin Eaters.
You could potentially argue that, even worse than any individual world lost in a Calamity, the worst thing an Ascian ever did was cause THAT world.