(Last night I attended a talk on St. Thomas Aquinas's thoughts on good and evil. These are things which occurred to me based on my notes and recollections)
Every human action has three components: the object, the intention, and the circumstances. A lot of bad moral reasoning arises from confusing the object with either the intentions or the circumstances. When we hear the word "object" we think of our goal, as in "What is the object of the game?" In the Church's terminology, object means our actual chosen behavior. It means what we actually do, not what we wish we could do. Modern people tend to think of themselves as ghosts in the machine, as omnipotent disembodied wills piloting their bodies like a meat robot. Contemporary moral thinking tends toward making sure the ghost in the machine has good intentions.
Consider a cake. The object - my actual chosen behavior - is eating a piece of cake. My intention is to enjoy a tasty dessert. The circumstances are it's my birthday. This action is therefore good. Let's change the object now. I devour the entire cake by myself, while the intentions and circumstances remain the same. I am guilty of the sin of gluttony. Consider a husband and wife making love. The object is they are having sex. Their intentions are to strengthen their emotional bonds and to conceive a child. The circumstances are they're in the privacy of their own home. The sex act in these circumstances is good. But what if this man and woman are not married to each other? Then it becomes the sin of fornication. Suppose the man and woman are both married but not to each other. They are both guilty of the sin of adultery.
In both cases - eating a cake or having sex - a good act is made evil by changing the object, intentions, or circumstances. All three components must be good for the act to be considered good. If there is a privation of what which is good, we call this evil. Darkness is not a thing, but the privation of light. Coldness is not a thing, but the privation of heat. Evil is not a thing, but the privation of goodness. Evil is a parasite. It only exists because the good exists. Without some degree of goodness in it, a thing could not exist at all. This means that the devil himself is good insofar as he has Being. All Being is contingent on God who is Being. Even the damned in hell have goodness because they have Being. C.S. Lewis thought the existence of evil in the world was itself a good proof of God's existence. You cannot call something crooked unless you know what straightness is. Likewise, you cannot call something or someone evil unless you know what goodness is.
The poet Dante Alighieri is known for his Divine Comedy, but only the first part - the Inferno - has really made an impression on pop culture. Dante portrayed hell as being divided into nine circles. The first few circles are considered to inflict the mildest punishments. Those are where the damned suffer for the sins of wantonness. They sinned through failing to control their natural urges: lust, gluttony, avarice, and wrath. The objects are not disordered, but rather through weakness sinners fell into excess or deficit. The lower circles of hell are populated by those who willfully and consciously choose evil: the violent, heretics, schismatics, sodomites, blasphemers, usurers, suicides, profligates, sowers of discord, sorcerers, and thieves. In the frozen wastes of Lake Cocytus, traitors are tormented by the devil himself.
Why are all forms of fraud punished the most severely in Dante's work? Man is made in the image and likeness of God. God is Truth. Telling lies is thus a direct assault upon the image of God that exists in all of us. It injures that likeness in ourselves and to those whom we tell lies. Good and evil always begins in the will. All of us suffer from weakness of will. But fraud involves a deliberate decision to commit an act of malice. Eating, drinking, and sex can all be good things. They can become evil if we misuse them. Lying cannot ever be a good thing. Granted, not all lying rises to the level of mortal sin. A white lie, by definition, does not involve grave matter. But the lie is never good in itself, and no intentions or circumstances can make an act which is evil in itself good. And before you ask, no, not even in everyone's favorite example of hiding Jews in your basement from the Nazis.