What is the ruling on one who argues that man's lack of infallibility from grave sins and the devil's portion in his heart prevent him from following the Prophet (peace be upon him) in everything, citing that the Prophet had the devil's portion removed from his heart and was made infallible from grave sins, and how is this explained in light of Moses' killing of a soul?

Allah commanded us to follow the Prophet, peace be upon him. Following him is possible in the fundamental acts of worship, such as performing the obligatory duties and refraining from forbidden acts. As for recommended acts, following him is possible in their essence, but not necessarily in their quantity or specific manner. The statement of Aisha, may Allah be pleased with her: "And who among you can do what the Prophet, peace be upon him, could do?" means the inability to follow him in the quantity and specific manner of his worship and devotion. Moses' killing of the Coptic man was not a major sin because it was not intentional, and a single blow often does not kill. His seeking forgiveness was either because he killed before being commanded, or for leaving what was better.