Kusakabe speculates that Sukuna's world slash may be due to a vow and later wonders why Sukuna isn't using his other abilities, which is frankly likely because Sukuna isn't ready to kill them yet. That said, it got me curious to see if there would be hints to what that binding vow is and I think I've found it. After using world slash, Sukuna ALWAYS followups with his cursed tool. These are all the times he's definitely used world slash and what occurred afterwards.
- World Slash against Gojo. After it kills Gojo, Sukuna's next attack is using the cursed tool against Kashimo.
- Sukuna launches World Slash at Kashimo. Even knowing that Kashimo has an immunity to lightning as he learned from instance #1, Sukuna then uses the cursed tool on Kashimo immediately before attacking with anything else.