Although Simba's flaw (sense of inferiority, living in the shadow of Mufasa) prolongs the central conflict (rejection of responsibilities and place as king), it does not directly cause Mufasa's death. Nonetheless, Simba's absence from Pride Rock indirectly caused the suffering of all of its inhabitants. Only when he lets go of his insecurities is he able to defeat Scar and reclaim the throne. Early on in the film, Simba tries to roar like his father, in an attempt to prove his strength and bravery. When he roars at the top of Pride Rock, he proves to himself that he is worthy of taking on the role his father once held. In this final moment of redemption, Simba officially accepts his position as king, restoring peace to the land, and overcoming his initial character flaw. Rather than trying to fill his father's shoes, he learns to draw strength from his spirit and forge his own path.
As a viewer, why is redemption so important? Films are entertaining because they absorb us into a world, involve us in its events, and take us on an emotional journey. The central vehicle for this journey is the protagonist. When the our "hero's" flaw is introduced, we become invested in the emotional struggle that stems from this flaw. Redemption is achieved when the hero overcomes their flaw and takes an action that demonstrates this transformation. If the hero has not grown by the end of the film, it feels like nothing substantial has happened, like the emotional journey was in vain, and that rooting for the protagonist was a waste of time. Moreover, the redemption of a hero gives us hope that we can change and grow as people (or teaches us a lesson, if the character's change does not directly result in triumph or a happy ending). You would be hard pressed to name a film where the protagonist does not undergo a substantial change. In reality, people don't change like they do in movies. However, we must keep the hope alive that we can learn from our mistakes in order to grow and make positive change.