“And Moshe said to the wicked one (rasha), ‘Why do you strike your fellow?’” (Shemot 2:13). The word translated “strike” (takeh) is technically in the future tense. From this our Sages derive that one who simply raises his hand against his neighbor is referred to as a rasha (a wicked person), even before actually striking him.
The prohibition of injuring another is biblical, derived from the verse: “He may be given up to forty lashes but not more” (Devarim 25:3). As is the case for all biblical prohibitions (lavin), a transgressor is liable to malkot (lashes) for transgressing, unless he is already subject to a financial penalty. Therefore, if someone causes an injury to another and the damage done is minimal (less than a perutah), he is liable to malkot. We might therefore conclude that someone who simply raises his hand against his neighbor (causing no damage and earning himself no financial liability) should incur the punishment of lashes. Why then is such a transgressor only referred to as a rasha but not lashed?
It is possible that the prooftext cited above is not the real source of the prohibition. Instead, it may be that the prohibition is rabbinic, with the biblical text simply serving as an asmachta (support). Even though according to this understanding the transgression of raising one’s hand against a neighbor is only rabbinic, someone who does so is referred to as a rasha. This status may disqualify him to serve as a witness, and may mean that his oath is not relied upon. Alternatively, it is possible that calling him a rasha does not disqualify him as a witness. It may simply mean that we are permitted to refer to him as a rasha, which is what Moshe did.
There is another significance to a person being considered a rasha. The person whom he is threatening is permitted to report him to the ruling authorities, Jewish or non-Jewish, and he is not considered a moser (an informer who turns in a fellow Jew to the authorities in defiance of Jewish law). Furthermore, the person being threatened is permitted to attack his attacker – not physically (as he has not yet been struck) but verbally, by name-calling. For example, he may call the threatening person a mamzer (a child born of an adulterous or incestuous union), even though doing so may cause his attacker more harm than the attacker would have caused him had he landed his threatened blow.