As traumatic as his childhood was, he already showed signs of celestial elitism. Expressing disdain for commoners and seeing slavery as normal. As an adult he is by far one of the most sadistic villains in the series. Finding death and pain to be very amusing. He expressed nothing but anger when he killed his father and brother, and while he may have felt a semblance of compassion for his crew, I believe he was capable of killing them at the drop of a hat all the same.
Having been twisted by the Kurozumi Family’s unfair persecution since his childhood, Orochi is an extremely cruel and oppressive tyrant who takes joy in making his subjects suffer as revenge for his family’s persecution.
Orochi is extremely domineering and unreasonable with absolutely no tolerance of anyone defying him or speaking ill of him, even if it is out of his earshot and in casual discussion. He is petty enough to mercilessly attack and kill anyone who offends him, whether they are a man, woman, or child (though may hesitate with a woman he has feelings for).
It doesn’t get much more evil than kidnapping children, experimenting on them, and eventually killing them as he planned. He has no care or loyalty for his subordinates, has no qualms about committing genocide through chemical warfare, released poison into air in a hissy fit, and was shown to be egomaniacal.
They are easily #1. Systematic oppression, humiliating and demeaning others, slavery, the erasure of history, they eclipses everything any other villain has done, bar none. We don’t even know how atrocious the events that occurred during the Void Century actually were, and we don’t know anything about Im or much about the CDs in general, so it’s likely they’re even worse than we’ve seen so far. And they don’t think they’re doing anything wrong.
*List by ExceedsTheCharacterL