Itachi is introduced as the man responsible for committing the near genocide of his clan, the uchiha’s, with only his little brother Sauske being spared. He also has a powerful Jitsu that lets him project illusions that cause great mental and physical pain on the victim, which he uses on Sauske twice to make him relive seeing Itachi killing his parents. He’s also part of the Akatsuki, essentially a ninja terrorist group
All this stuff in the original series paints him as a really evil whose a clear cut villain
Then in Shippuden its revealed that he really did love his clan, but his clan was planning on staging a war against on another village (Konoha) and were basically the ninja version of the Nazis. Itachi then decides to leak info of the clans activities to Konoha, in order to prepare them to stop the war. This results in one of the higher ups in Konoha (Danzo) killing Itachi’s best friend to gain his power, then makes an offer with Itatchi that he will spare the slaughter of both his own and Itahi’s village as long as he kills his own clan, which Itachi aggress to since he says he’s fine with being seen as a monster as long as no more war happens. It’s also revealed that he was a double agent for the Akatsuki too, sine he saw them as a threat and was secretly leaking info on them to his village
Comparing how he acted in the original series vs how he acts in Shippuden, a lot of people feel like the “Itachi was secretly good all along” twist was a giant retcon