Molyneux's definition of "law" is a good one: "Law is an opinion with a gun"; but this one is more succinct and accurate:
Law is an excuse to do harm. (DBR)
Law, after all, does not wield the gun, and the ones who write it do not wield it themselves; they write it and codify it. Written, without any force to back it up, it is merely an excuse to do harm: to chase someone, invade their home, bind them, kidnap them, and lock them up for an extended period of time in a facility where they are likely to be killed, raped, or otherwise harmed by other inmates.
Many, if not most law and regulation is for "victimless" crimes or activities. If there is no victim, there can be no crime; "the state" is not a person, and convenient appeals to "society" fall on deaf ears. They are merely excuses to harm someone for peaceful activity. Most drug laws and traffic regulations fall into this category, as do licensure laws. (One might say, "But if an unlicensed doctor practiced on someone, he could do harm!"; but it is wrong to do harm to someone for mere distant potential harm they might do; it is a free individual's right to enter into the risks and contracts they choose.)
What of "good" laws, i.e., laws about actual harmful activities? Even then, the harm threatened frequently is unjust, i.e., does not call for restitution to the victim (insofar as is possible) and equal retribution to the criminal; instead, it demands arbitrary penalties, and any payments go to the state. So when we eliminate victimless laws/regulations, those with non-commensurate penalties, and repetition, we are left with very few, if any laws; I acknowledge it is possible that there may be one or two such laws, although I can't think of any.
One does not need arbitrary decreed "law" to know that killing is wrong and that the rightful penalty is to do the same to the killer (they may negotiate for something else with the survivors if they can reach agreement, such as payment or incarceration: that is up to the victim or heirs).