I revisited an AMV from a handful of years ago, and saw the creator make a comment like ‘Sasuke is so stupid he just keeps believing anything people say to him’ concerning like how he feel in with both Orochimaru and Madara, and just---
... I wonder if people like that really watched the same series as I did jdsogkodsgko
Cause in both instances, his insecurities and fears and trauma were preyed on in order to manipulate him. With Orochimaru, all he had to do was cause his trauma to resurface and be like ‘remember when you were too weak to save your family? I can change that and help you find peace’ and coupled with that encounter with Itachi, Sasuke was in the palm of his hand. With Madara -- if anything, Sasuke believing Madara was far from a stupid move.
Cause not only is this another Uchiha, but this is an Uchiha who comes carrying history and information that Sasuke never knew about. And he tried to shut him down by saying he was lying, but he literally provided all the context to tell him that what he was saying about the genocide and Itachi was the truth. With, effectively, shattered his world and led him into his ‘infamous’ mental breakdown.
Sasuke is far from stupid. It’s just that when it comes to his trauma, tied in with his family and the clan in general, it’s easy to sway his judgement and then manipulate him. There’s a reason Itachi said he was a ‘blank canvas’ that people keep trying to paint in their colors. We won’t mention how he was doing the same thing, but--