Harm No. 4: An Action of Stupidity
Hazrat Hakeemul Ummnat Thanwi (رحمۃ اللہ علیہ) said: "Every sin is a sign of stupidity and foolishness. Anyone who commits sin, then this is a sign that there is some deficiency in his intellect. He is disobeying such a Being in whose hands is our life and death, our health and sickness, peace and comfort, good and evil end. If his intellect was sound he would never commit sin.
As for the one who casts evil glances, Hazrat says that he is extremely foolish. By casting glances he attains nothing other than restlessness in his heart. By looking at a beauty, one never attains him/her, rather one gets uneasiness and the heart becomes restless when one remembers him/her.
Allah Ta'ala has granted me a new aspect of knowledge. To cause inconvenience to any Muslim or to bother him in any way is HARAAM. Now that person who is casting evil glances, is he not a Muslim? Is he not causing pain to himself and making his own heart restless? Therefore, the same way it is forbidden to cause pain and harm to any Muslim, it is likewise forbidden to cause pain to one's own heart by making it restless and uneasy.