If one hits another Jew, they are violating a lo ta'ase, a negative commandment of the Torah. Even if a person does not hit, but just raises his hand to hit, he is considered a rasha, wicked. Such a person may not be counted in a minyan and is considered excommunicated by a cherem hakadmonim, an excommunication by the early authorities.
Even when an individual started an argument and shames another, raising a hand to hurt them is prohibited. Even if the other person hits first, it is forbidden to hit back. However, if the attacker continues hitting, it is permitted to hit back if it is the only way to protect oneself.
Rav Yosef Yeshaya Braun, member of the Badatz of Crown Heights, One Minute Halacha, #154, reprinted from crownheightsconnect.com