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Aquinas' view of natural law
In the thirteenth century St Thomas Aquinas saw in Aristotle’s philosophy a rational foundation for Christian doctrine. In hisSumma theologica and other works he set out, systematically, a new approach to the philosophy of the Christian religion, based on Aristotle, which saw all aspects as interconnected and necessary, to all other aspects. Like one of the great Gothic cathedrals which were being built as Aquinas wrote, he saw the great weight of the Christian concept of God as being distributed equally onto many pillars of doctrine, among them analogy and the specific interpretation of terms such as omnipotence, omniscience, omnibenevolence and the interpretation of scripture, of the relationship between faith and reason, of natural law and morality. Each doctrinal pillar was integral to the structure of Aquinas’ philosophy: undermine one and the whole edifice is in danger.
From Aristotle, Aquinas drew the belief that human beings can use reason to discover how they should behave, to appreciate the existence and the nature of natural laws and the human ability to choose to follow them and flourish, to go against them and fall short. While Aristotle was noncommittal about the existence of God as creator or sustainer, for Aquinas this had been revealed through Christ and the scriptures, therefore using reason to understand natural laws was using reason to gain insight into God’s will. It was possible, for Aquinas, for a heathen to live a naturally good life, though as might be expected from a Catholic writer, he believed Christian education, baptism and sacraments are necessary for a person to achieve salvation. Like Aristotle, Aquinas understood goodness as something fulfilling its nature; for Aquinas this meant doing God’s will in being what he created it to be, fulfilling its divinely willed potential.
Living well
For Aquinas, reason dictates that a good life is lived long, peacefully, productively, in the pursuit of wisdom and with a commitment to passing it on to the next generation. He only differs from Aristotle in seeing the necessity of religious practice, seemingly as part of philosophy, in allowing people access to the revealed truths that are necessary for salvation and encouraging in them a sense of awe and humility, of thankfulness for the gifts of life and reason, rather than suggesting pride in human achievements. 
Think about it: a dead person cannot be a good person. They cannot fulfil any aspect of their human potential to move, think, act or decide. Life must therefore be the first requirement for goodness and a person who is alive cannot be all bad because they are still a person, fulfilling some of their divinely willed potential if not any of the aspects of that potential that are under their control. This supports the Christian belief in the sanctity of life; even Hitler or Pol Pot are human beings and worthy of respect and love as such. We all deplore their moral choices but fundamentally human life is still life and is an inherently good thing, not to be destroyed lightly.
Fulfilling potential
Aquinas characterises the qualities of a fulfilled existence: life, peace, prosperity, procreation, philosophy, praise, pedagogy as the primary precepts of natural law. Some people may be prevented from leading such a life by a physical or mental impairment, or for some other reason beyond their or anyone else's control. Obviously they are still good and incredibly valuable, even if we regret the "natural evil" that prevents them from being all that they might wish to be. Aquinas suggests that such natural evil is an occasional falling short in God's creation, allowed by the creator in order that the existence and nature of true goodness may be facilitated, understood and appreciated.
A person naturally prevented from fulfilling human potential in some aspect can always try to compensate in other aspect; somebody who for some reason cannot have children of their own could find fulfilment through adopting and giving a home to a child who has no parents; or they could find fulfilment by devoting themselves to study or business or teaching, perhaps being more able to succeed because of the single-minded application they are able to give than those who have families to balance with work.
Indeed, sometimes fully actualising human potential in one aspect requires focus and concentration which may be made easier by choosing not to pursue another aspect. For example, Catholic priests and monks are expected to devote themselves to prayer, study and working to make life better for others. Pursuing worldly wealth and having families tends to get in the way of this. Jesus himself chose to remain single and to abandon the family business in order to spread God's message. In the Middle Ages, around the time that Aquinas wrote, the Church began to enforce celibacy for all priests and to prevent them from engaging in many other kinds of work they were doing, from medicine to soldiery. This was justified, according to Aquinas, because the conscious choice of some people not to actualise some aspects of their potential made human beings in general more able to be fulfilled.
Aquinas' characterisation of a fulfilled existence - life, peace, prosperity, procreation, philosophy, praise, pedagogy - are the ends which all naturally good actions will serve and the ends which no naturally good action will prevent or hamper. It follows from this that some types of actions will generally be categorised as positive and some negative. Murder will always curb the "potential" of someone living and therefore is naturally evil, always wrong. If a primary precept involves achieving full potential, it is a secondary precept of natural law that murder is wrong because murder prevents someone from achieving their potential.