How can Simba believe he was responsible for Mufasa's death??
1) I mean there was no possible way he could've caused or stopped the stampede, it wasn't his fault.
2) There no possible way that Mufasa being unable to save himself was Simba's fault.
3) Little Simba's roar sounded like a frightened house cat, it wasn't intense enough to scare the Wilderneasts.