At what point do you think Kishimoto had already decided on the truth about Itachi and the Uchiha clan massacre?

To me, it's quite obvious that since the flashback of the Uchiha massacre that happens during Naruto and Sasuke's fight in the Valley of the End, he had already made up his mind about it.

To me, this becomes obvious when you observe some details. The information that the Uchiha were Konoha's military force and all the focus that this receives, Fugaku wanting Itachi to join the ANBU and saying that this would be beneficial for the clan, Itachi losing his temper with his father's assistants and saying how they were idiots for being bound by the clan's shackles.

Along with all of Itachi's behavior with Sasuke, which would be very strange if he had simply gone crazy out of nowhere or just pretended everything since they were children.

All these points already open the door for a larger story to unfold. After all, if Itachi was meant to be a crazy madman who killed the clan just for power, why include these details in the story?

And you, at what point in the story do you think Kishimoto actually conceived of Itachi?