2. I was thinking about this option, i'll explain good
Tobi (or madara or robert de niro i don't know) killed the entire clan while itachi was off for a mission and sasuke was away. Itachi arrived first and he saw the uchiha ppl and his parents death. Sasuke come and watch itachi near the corpse, in that point itachi think to use that moment to make the skinny sasuke much stronger fillin him with hate and he pretend to be the real killer and used the mangekyou on his brother to make him belive that with an illusion genjutsu. Sasuke full of hate because his brother became stronger enough to kill the real assassin (everybody know that the sasuke sharingan is powerfull respect the itachi's one) this is my small silly opinion

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