What would have happened if Itachi decided to side with the Uchiha during Coup d'etat

[UnknownJin]

Itachi would kill them all lol

 

[Red Slayer]

Uchiha would get stomped very hard.

 

[Gabby-chan]

Plot is the strongest existence in Naruto. It is completely invincible and can kill whomever it pleases. Plot killed the Uchiha.

 

[Genetics]

Itachi would have easily succeeded ONLY IF he got Obito to help him

 

[MakavelitD]

It would've been better for Konoha to be ruled by maybe a combo of Itachi + Shisui than the inept and unable failure called Hiruzen and then Danzo. I don't think there even needed to be a lot of bloodshed, Itachi's and Shisui's genjutsu prowess would've been too much for even the strongest fighters in Konoha.
Itachi could control multiple fighters at once and so could Shisui, add two Susanoo(I'm guessing an MS Shisui also had one, following the logical pattern) and it would be just too much, I think Hiruzen would've surrendered on the first day. If the Uchiha won, maybe Obito would've also returned to Konoha.

But yeah even without Shisui, Itachi could carry

 

[Nidaime Suiton]

Danzo ask to orochimaru to create a Virus which kill all uchiha.

A virus which work only against sharingan possessors.

And he can revived Tobirama with Edo Tensei, specialist in Anti-Uchiha science , to help him in his research to destroy uchiha clan.