Dude.... do you not understand that he found obito before killing his clan, before leaving the village... everything didn't just happen in one day.... he had the mission to kill the clan for a while.. but the village tried other methods.. He made a deal with tobi so that akatsuki could not make any moves on Akatsuki... and temporarily joined..
Orochimaru attacked him , Itachi defeated him with 3 tomoe.. all he had at that time....
After all Hiruzen's negotiations faied , and Shishui failed (because of danzo), Shishui gave Itachi his eye and died... this would have been around age 12...
It would have been around here when anbu's started pestering Itachi to hurry up and do the mission...(We saw when one of them came to him and told him if he doesn't do it , they will)
Itachi becomes anbu captain...
Itachi goes back to tobi and asks him for help to kill the clan..
They both do it..
Itachi talks to hiruzen about sasuke
Itachi leaves the village , permanently ,
Itachi partnered with Kisame (Itachi activates mangekyou) showing he has it
Sasori is still partner less
Deidara recruited into akatsuki and becomes Sasori's partner
Isn't that difficult of a time line.....Your problem is you believe the entire massacre thing, shishui death , hiruzen negotiations all went down in a week or day.... it was a drawn out thing... and Itachi became a spy for konoha almost the moment he joined anbu (age 10)
Which why when the 3 uchiha confronted him, they said that after only half a year (10.5) of him joining anbu he started acting different... He was already a spy.. already had the mission