What is the ruling on one who used to raise his voice at his parents due to anger resulting from a neurotic anxiety disorder, and now that he has recovered, how can he atone for that?

It is not permissible to raise one's voice at one's parents, as this contradicts righteousness, kindness, and the gentle speech that Allah Almighty commanded in His saying: "So do not say to them [even] 'uff' and do not repel them but speak to them a noble word." However, if raising one's voice results from a psychological illness and without intention, it is hoped that one will not be held accountable for it. After recovering from the illness, the person must treat their parents righteously, show them kindness, and lower their voice in front of them.