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
I think he is the main cause of this and caused a lot of trouble during Naruto and Sasuke fight in part 1 and in part to the right before he died Naruto says he'll leave everything to Sasuke right before he could destroy that leave himself
I said meeting up with the touch as the main cause of this and after an episode 84-86 he along with the Akatsuki Group want the nine tails from Naruto and having sasuke to turn against him and that's why Kakashi stop them from fighting