I don't know. Voidborns feel to me like tyranids, not necessarily evil but savage. They don't understand human nature as Nocturne or Shaco does. And he is pretty outdated. I don't how would his perspective would change. I would see him as a equal of Vel'Koz after an update.
Still, If I was facing one of them, I would be terrified and definitely descripted them as the most evil being I ever seen before I die. :P But in generally, from the perspective of a reader, they feel like like ''meh'' compare to others.
Nocturne has sadistic pleasure for shattering someone's mind inside and torn apart their life like worthless, pathetic bug beneath your feet, because you're that worthless except being delicious.
Shaco pranks you to death, literally, because It is funny.
Thresh, no word is needed.
Mordekaiser. That would be arguable If he wasn't Thresh like, since he also tortures the souls forever. Without it, I would say he has ''logical'' goal compare to others. But no, he is sadistic, malicious and merciless.
Xerath...Bloodline has made him evil more than ever he has been. Burning people alive, hearing the screams and having sadistic pleasure? If It is not what Nasus saw, he is definitely top 5.
There are honorable mentions like Tahm Kench, Evelynn, Brand (maybe, not sure about his nature), Singed etc but I still think champions above are more evil.