Is a person's feeling of happiness at another's misfortune - while striving to resolve it - considered a sin for which one is held accountable, or is it merely from the whispers of Satan?

A Muslim loves good for his brother and does not rejoice at any affliction that befalls him. If his feeling is merely a passing thought that he neither speaks of nor acts upon, then there is nothing wrong with it, for the hadith states: "Indeed, Allah has forgiven my Ummah for whatever they contemplate in their hearts, as long as they do not speak of it or act upon it."

The envious person whose envy does not lead him to any bad behavior towards the envied one incurs no sin, especially if what prevents him from doing so is his piety towards Allah. It is sufficient for him in his struggle against himself that he does not act upon these inner thoughts nor resolve to act upon them.