Even after accepting that the Uchiha coup was inevitable, Itachi wasn’t hellbent on fighting them. It wasn’t until Danzo offered Itachi the mother of all ultimatums that he agreed to become a villain. He could either die along with the rest of his clan at the hands of the Leaf Village, or he could do the dirty deed himself, sparing only his younger brother, and walk away alive. As we already know, Itachi took this deal.
A lot happens during Itachi’s massacre, but the gist is that Itachi performs his duty relentlessly and without mercy. As a shinobi prodigy, Itachi stood as one of the Village’s strongest soldiers even as a kid. When he happens upon his parents, the two submit and allow him to kill them. They know why Itachi has agreed to kill his clan, and they are proud of him despite their differences. Sasuke appears, and witnesses the death of his parents before Itachi leaves, encouraging him to get stronger and seek revenge.
Prior to killing his parents, Itachi also encounters a man claiming to be Madara Uchiha. The two men then agree to split the killing up between each other. This partnership then leads Itachi to join Madara’s organization, the Akatsuki. For the next few years, Itachi lived on the run as an outcast and a war criminal before Sasuke finally tracked him down for their final confrontation.