Board Thread:Naruto Gaiden/@comment-29393962-20161105042959/@comment-2006510-20161105210950

I'll assume you're asking how Itachi used genjutsu - specifically Tsukuyomi - to slaughter the Uchiha clan, in reference to 'Itachi Shinden: Book of Dark Night' until said otherwise.

The reason is simple: Tsukuyomi causes the victim to experience time passing at a rate unlike reality. The result? Itachi uses Tsukuyomi to make everyone experience a lifetime's worth of time passage - pleasant or painful - in point many-zeros of a second. That causes an immense strain on the victim's brain, trying and failing to process so much so quickly; this causes their death, rather than any direct effect of the genjutsu itself

So in effect, their own minds technically killed them