JUJUTSU KAISEN S01 E29 Premature Death

"A boy fights... for "the right death.""

August 2007. Gojo starts to handle missions on his own, causing Geto to, as well. One day a certain individual appears before Geto. What's the conclusion that led to Gojo becoming the strongest jujutsu sorcerer, and Geto becoming the most evil curse user?