So, Simba did confess to being responsible for Mufasa's death, but also made clear that it was an accident. Given how desperate the Pridelanders were to get out of the bad situation with Scar as king, I'm surprised that they would have let Scar murder Simba (if Simba hadn't fought back or was unable to fight back, they'd have watched their last hope be thrown to his death.)
Also, if they were dumb enough to believe that Simba really conspired to murder Mufasa, even as a cub, why would they believe a confession of Scar that he killed Mufasa when Simba had knocked him over and was nearly strangling him?