Is it true to insult a non-muslim?
Islam is against all kinds of evil deeds and hurting, because Islam tries to honor man. It helps him find his real values. For this reason, Islam gives orders which make man reach every kind of beauty and maturity and prohibits every kind of bad and ugly deeds.
We can say based on those two basic rules that every kind of deed called insult and curse, which makes people hurt and offend is prohibited and sin because it is prohibited to hurt Muslims and it makes people sinful. If a non-believer is innocent, it is forbidden to disturb him or her in Islam because our Prophet (pbuh) said that “Whoever hurts a dhimmi should know that I am her or his enemy.” (Abu Yusuf, Hara, Matbaatu's Salafiya, 1397 h. Cairo, p.135)
Whatever the condition is, it is not allowed to insult or curse. It is not proper for a Muslim to insult a person no matter if he is an unbeliever or a tyrant them. It does not befit a Muslim.