They were idiots. That’s why they were killed. They were stupid enough to allow themselves to be hunted down and killed by the Empire. And that’s all just relying on information from the OT.
The only thing that the OT says is they were hunted down and killed. It says nothing about them being idiots.
One can infer that they were idiots for not sensing betrayal until it was too late, by using evidence from the OT. Their betrayal happened quickly enough that Anakin had children which Obi-Wan knew about but Vader did not. Obi-Wan and Yoda in ANH and ESB have great power, and yet the Jedi Knights—Obi-Wan and Yoda included—were there to see these dark events unfold but be unable to do anything to stop them, despite their powers of persuasion, perception, distraction, et al… The Jedi Knights are painted in a retrospectively-positive light because Ben Kenobi tells us about them, and he has positive feelings for them. That doesn’t make them actually as wise as he remembers them to be.