#jujutsukaisen manga spoilers

Nanami had identified how initially Yuji & Mahito were “children” at the start of their development.

Now, having been killed by Mahito & passing his responsibility to Yuji, his death marked the ending of their “childhood” stages.

in other words.. Nanami was a figure influencing both their milestones of early development at the beginning of the story.

by killing Nanami, Mahito confirmed his growth as a more mature cursed spirit;

and inheriting Nanami’s burdens marked Yuji’s further growth into an adult.

in many ways, Nanami had been the closest person to a “father figure” helping Yuji reconcile what it means to bear the responsibility of a sorcerer.

His death is symbolic for how Yuji must now move on to the next stages beyond “childhood”; and what a lonely feeling it can be.