People are also forgetting that every person with the same CT won't get the same technique usage, for example Yuji using Sukunas CT is way different to Sukunas usage of shrine.
It's every negative emotion a person could feel that creates cursed energy, also if committing horrible acts or sining equats strength their would uncountable special grades not to mention the really bad cursed users in jjk, Kenjaku would be the strongest in the verse.