There is natural law, which includes the substance of the Ten Commandments, marriage, and so on, and perfect law, also called the new law or the law of the Gospel.
Saint Thomas Aquinas discusses this in the Summa Theologiae (I-II, qq. 90–108, the “Treatise on Law”), and it is also mentioned in the Epistle of Saint James.
James 1:25 But one who has looked intently at the perfect law, the law of freedom, and has continued in it, not having become a forgetful hearer but an active doer, this person will be blessed in what he does.
When marriage was raised to the dignity of a holy sacrament, it was perfected. Yet we would still say that a consecrated life of celibacy is a higher calling. Therefore we would not be violating natural law by remaining celibate, since it is preferred in the Gospel, and ultimately those in heaven are celibate. We can apply this same reasoning to capital punishment and say that it was an imperfect but necessary form of punishment. It would not be intrinsically evil, as it forms part of natural law, but it can reasonably be considered inadmissible now that human dignity can be preserved through alternative forms of punishment. We possess infinite human dignity by virtue of being made in the image of God. This is why we have an immortal soul, and that forms part of perfect law. This is my understanding so far, and I am still studying the subject as much as possible.