But as he got older, I don't understand why he didn't start questioning what Scar told him, since he did what Scar told him to do (roar at the lizard), and then Scar rudely told Simba to run away and never return after Mufasa was killed.
If Simba had even the slightest bit of skepticism, he could have told Nala that Scar told him that Mufasa's death was his fault. She would have definitely assured Simba that Scar was lying and gotten rid of that awful guilt that Simba felt. Simba should have definitely been more skeptical as Nala told him about the awful things that Scar did. And yet, even as Simba was ready to rip Scar apart, he still believed Scar's lie

Simba should have been more skeptical before Scar finally whispered the truth
