Will a person be held accountable for what goes on in his mind and what his soul urges him to wish evil upon others, even if he seeks forgiveness and repents?

Wishing evil upon a Muslim and envying them is forbidden by Islamic law. The evidence for its prohibition from the Sunnah is what Muslim narrated from Abu Hurairah (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: "Do not envy one another, and do not bid against one another, and do not hate one another." The evidence for its prohibition from scholarly consensus is what Imam Al-Nawawi mentioned: that the texts of the Sharia and the consensus of scholars affirm the prohibition of envy, belittling Muslims, and wishing harm upon them.

As for holding a person accountable for such thoughts when they are not acquired actions, this refers back to their preceding and subsequent causes. The heart does not wish evil upon others except due to envy and hatred, and that is because of indulging in its forbidden causes and neglecting its deterrents. All of this falls under accountability and free choice. Thus, accountability for envy and its consequences stems from its preceding voluntary causes that bring it about, and its subsequent lack of repentance and failure to combat its effects. A Muslim must strive to rid themselves of envy. The cure for envy lies in understanding its causes and treatment, and working towards that so that there is no malice in their heart towards any Muslim. This was mentioned in the story of the man whom the Prophet (peace and blessings of Allah be upon him) described as being among the people of Paradise, the reason for which was that he harbored no malice or envy towards any Muslim.