If there were no laws prohibiting murder or rape, would those acts be justified?
This is a question that cannot be answered in an objective way. Any answer to this question is based on a given individual’s ideas about justice. We can be sure in our own minds of the answer, but we cannot provide scientific proof that we are right.
In the minds of most people, murder and rape would not be justified even if there were no laws against them. This is because both murder and rape go against what we in the West perceive as natural law. We believe that there are laws that are higher than the laws of any particular government. We believe that governments that do not follow these laws are, at least to some degree, unjust governments.
One formulation of this idea was famously given by John Locke. Locke argued that all people have, by virtue of being human, certain rights. These include the rights to their life, liberty, and property. Here, we can see why murder and rape are wrong. Murder, of course, deprives a person of their life. Rape deprives a person of their liberty to do as they wish with their bodies and their liberty to not be coerced into doing things.
Because murder and rape are contrary to natural law (at least in our Western eyes), they are unjustified regardless of whether there are actual governmental laws banning them.