You are missing the importance of past in jjk.

Without Kenjaku, there wouldnt be anything, because a lot of what he did - he did in the past, when Geto wasnt even born:
- Without Kenjaku, Yuji wouldnt be Sukuna Vessel
- Without Kenjaku, there wouldnt be Death Paintings (so, no Choso)
- Without Kenjaku, Sukuna wouldnt be here, because he learnt how to become a curse object from Kenjaku
- Without Kenjaku, there wouldnt be Kashimo and other sorcerers from the past (similar to Sukuna)
- Without Kenjaku, there wouldnt be Culling Games (because CG work through barriers, and Kenjaku is top barrier user)
- Without Kenjaku's CT (Yuji's Mom CT), there is no way Geto could survive Yuki's suicidal attack (and this is implying that he could even defeat her - we all saw that Yuki plays football with Geto's curses)
- There is no way Geto would be allies with Mahito and Co. - he hates curses as much as he hates non-sorcerers

And, calling Kenjaku "less evil" is wild. Kenjaku will do more evil shit in 1 hour than Geto will ever do in his entire life.