I get that it is confusing but unfortunately the rule is necessary because without it the category will be filled with villains who attempted cataclysm, genocide, or omnicide and are Pure Evil because of that but qualify because they didn't succeed.

And it goes against the point of the category of villains that managed to be Pure Evil despite having not killed anyone and having committed a different heinous crime to qualify. As it is possible for villains to qualify with just that.

Yeah but I mean villains like King Boo. He never killed anyone and isn't under the category just because he attempts omnicide. But why if he never succeeds in it?

And also, not every villain would be under there. There are not many villains who try to commit genocide or omnicide and haven't killed anyone before.