It is very simple Indra recarnation are very power hungry and emotionally unstable too bad characteristics when you're the leader of the clan and look to for guidanceWhy are the Uchiha so easy to manipulate? Despite their power and being genjutsu experts, they are really easy to exploit.
Black Zetsu managed to manipulate Indra and Madara.
Orchestrated the events to destabilize any chance of peace the clan had due to the tablet being changed.
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Zetsu basically made the uchiha his tools since their creation.
Then there’s:
Danzo manipulating Itachi
Itachi manipulating Sasuke