What is the ruling on one who desires to commit sins but refrains from doing so out of fear of Allah? Is he sinful like the one who commits them, and how does he expiate his sin?

Mere lust and inclination towards sin do not incur sin upon a person unless they speak of it or act upon it. Allah has forgiven this Ummah for what they contemplate within themselves. And whoever strives against their soul and abandons sin out of fear of Allah and hope for His reward, a full good deed is written for them, and Allah guides them to the paths of good. As Allah Almighty says: "And those who strive for Us – We will surely guide them to Our ways. And indeed, Allah is with the doers of good" [29:69], and "But as for he who feared the station of his Lord and prevented the soul from [unlawful] desire, then indeed, Paradise will be his refuge" [79:40-41]. If one intends to commit an evil deed but does not act upon it, Allah records it as a complete good deed.