Ahadi names his firstborn such to hammer home to everyone that this lion, and this one alone, shall someday rule the Pridelands. Mufasa's entire childhood revolves around being trained to take his mother's place someday, and so he never gets to have any play-time or real childhood at all. Due to this, he grows very frustrated with his lot in life, and very jealous of his younger brother, who gets all the free-time in the world. Frustration turns to anger, and anger into resentment and almost hate -- until Mufasa despises his sibling. This deep-seated anger issue turns fatal when -- out of rage over Taka taking Sarafina from him -- Mufasa kills Miitu.
But of course, Ahadi would never let that happen