Subjectivity of Justice

If a legal system bans capital punishment, then it must not make exceptions, even for exceptional cases.  No matter how heinous a crime someone commits, then they cannot legally be executed.  The reason for this is that making even one exception to the law invalidates every case in which the law should apply; as there is no objective way to determine exceptions, breaking the law (and it would be breaking it) in order to fulfil a subjective ideal of justice is radically inappropriate.  After all, if the legal system makes an exception once, why would they not do so again?  It is easy to say that they will only make exceptions in cases where capital punishment is justified, but who exactly determines what constitutes justification?  One could suggest a codified definition of exceptions, but in that case they would no longer be exceptions, they would simply be part of the law!  As such, the law would not in fact be banning capital punishment, but merely making very specific circumstances in which it would apply.