Does one who utters the word "kafir" (infidel) to his brother, intending to deter him from an evil deed, but not intending to excommunicate him, become an infidel thereby? Is he sinful for that? And how should he repent?

A Muslim must change the evil with his hand, and if he is unable, then with his tongue, and if he is unable, then with his heart, while it is necessary to be gentle and lenient in all matters, and to avoid excommunication (takfir). Allah Almighty commanded Moses and Aaron to speak gently to Pharaoh, and the Prophet, peace and blessings be upon him, warned against excommunicating a Muslim. The word "kufr" (disbelief) in the hadith is interpreted as minor disbelief, which is one of the major sins. Repentance from this sin is like repentance from all other sins.