Does Moses's (peace be upon him) lack of fluency in speech and his killing of a soul conflict with his infallibility from moral and physical flaws?

Allah honored the prophets and chose them to convey the message, perfecting their character and disposition. The heaviness in Moses’ tongue was an accidental defect caused by a burning coal he put in his mouth when he was young, and this defect was removed by his supplication to Allah. As for the incident of his killing the Coptic man, it was an unintentional mistake that occurred before his prophethood, and its purpose was to aid the oppressed. Prophets are infallible from major sins, both before and after prophethood. Minor sins may occur from them, but they do not persist in them; rather, they repent from them.