With freedom comes responsibility. We are responsible for our moral behaviour because we are made in God's image as rational beings, capable of knowing what the morally right and good thing to do is and as free beings, capable of choosing to do the morally right and good thing. These two capacities, to know and to choose, together form what is called conscience. Loosely translated, conscience means 'with knowledge'. In other words, when we make moral choices, we make them based on what we know about the goals we want to achieve, the ways or means to achieve them, the circumstances in which we need to achieve them and the consequences of both the means we choose and the outcomes we achieve. When we have weighed all these things, we make a judgment based on our knowledge of what the morally right thing to do is. We are then obliged to follow our conscience and do the morally right thing, taking responsibility for our decision.

Sin is not simply about choosing to do something bad or evil. It is always about faulty judgments and subsequent flawed choices about what is good and right; sin is missing the mark. This can happen in two ways. Sometimes it is about choosing to achieve something what we think is good, though in reality when all is considered it is actually bad. Sometimes it is about choosing to achieve something that really is good, but doing so at the expense of things that are really greater goods and therefore should take precedence. For example, the murderer chooses to kill someone. But when he or she kills another person, that action is motivated by some mistaken understanding of a good thing that he or she hopes to achieve through the murder. Or maybe there is some really good thing that he or she hopes to achieve but at the expense of something that is a greater good in this case, the life of an innocent person. Consider the first case. A person might kill another human being in the belief that this achieves justice. But this is a mistaken understanding of justice. Or, as a second example of the first instance, perhaps the murderer is doing it because he or she actually desires to be 'evil', and believes it would be 'good' to be 'evil'. For the murderer, being 'evil' is a 'good' thing, though in reality this, of course, cannot be so. As an example of the second case, perhaps the murderer really does get some pleasure out of killing other people. Pleasure is really a good thing, but it is wrong to choose it over, in this case, the life of another person. Justice, pleasure, relief, and even 'evil' in this sense are all 'good' things in the mind of the murderer, because they are all things that are desired by the person acting, by the murderer in this case. But the murderer's actions remain objectively sinful, and remain morally wrong. They can never be morally right, regardless of the 'goods' that the murderer sees in them, be they real goods like pleasure, or imagined one's like 'being evil'. And this is so because the murderer cannot objectively defend the notion that any of those good or supposedly good things provide sufficient reason for wilfully killing another person. This is especially the case since the murderer's subjective perception of the good thing itself is mistaken or 'disordered' (such as wanting to be evil, or believing that killing someone achieves justice, or believing that one's own pleasure is a greater good than the life of another person). For more information on how the Catholic Church works out what is objectively morally good, see the section on Natural Law.

The severity of the sin, of the wrongdoing, is traditionally divided into two categories, venial sin and mortal sin. Mortal sin is the more serious of the two. Mortal sin is a conscious and freely chosen turning away from goodness, from the ultimate good and from God. It is a free rejection of our capacity to love and of our capacity to seek and find the truth. Mortal sin is a rejection of what we know to be the right thing to do in our conscience. In other words, it is a rejection of our very own dignity as beings created in the image of God. Such sins are called mortal because, in the language of the tradition, mortal sins lead to death and eternal damnation in Hell, unless the person sincerely seeks forgiveness. But one doesn't need to believe in Hell, or damnation, or eternal punishment to realise that the kinds of wrongdoings that are called mortal sin can have very serious and sometimes really deadly consequences for the flourishing of the whole community. For example, in the case of murder, in addition to the death of another person, there is significant trauma associated with suffering, anger, resentment, mistrust, fear, financial burdens and costs, disruption of work, destruction of families, and so on. There are specific conditions that must be met for a sin to be a mortal sin. First, it must be done with full knowledge; second, it must be freely chosen or willed, and third, it must concern grave or serious matter. Grave matter are those things that concern a fundamental good of the human person. Grave matter includes unjustifiably killing or physically harming a person, taking something that does not belong to you, having sexual relations with someone who is not your spouse, and withholding the truth from or deceiving people who have a right to certain information from you. In other words, a sin is only mortal in the strict sense when you freely choose to do it, knowing that it is the wrong thing to do. Of course, we are also very good at deceiving ourselves about what we really know, or finding ways to justify our behaviour to make it look like we didn't know or didn't choose. Being really honest with ourselves about these things is essential if we are going live whole-hearted, morally responsible lives. Whether a sin meets the strict definition of mortal sin or not intentions and actions must be wrong when they break down the kind of just and loving human community that God desires for us and that we really should desire for ourselves. Such actions break down the kind of community that we described in the previous section about Love, being free and being responsible means avoiding doing the wrong thing and trying sincerely to do the right thing for ourselves and for others.