Many texts can be difficult to read because one does not have a common understanding of the language used by the writer. This fact of language hinders communication of all sorts. Since the meaning of words is ultimately decided by how they are used, there is a somewhat circular reference for most precise words. For example, the English word "murder" has a traditional English (from English Common Law) definition of "unlawful killing, with malice aforethought, of another human". Note, the single word contains several elements:
- Unlawful: it must be against a law
- Malice aforethought, one must with malice specifically intend to commit the act
- Another human: it can only able to be committed by humans against other humans
If any element is lacking, then it is not "murder" according to a nearly universal use of English for centuries. Murder is then almost necessarily a word which describes a mortal sin because the word itself includes the intent and knowledge of a specific act.
This narrowly restricts the word "murder" in meaning. State sanctioned killings of humans (in war, in criminal systems, in self defense, etc), deaths of others caused by mistakes, negligence, ignorance, and the like, and deaths caused by natural, animal causes are not "murder". There is a term for the general concept of a human causing the death of another human, that is "homicide". And there are legal definitions which may vary across jurisdictions concerning specific types of homicide. But this is an illustration of the problem we face in common, especially when reading, writing, or discussing sin.
Murder is a rare sin for most as a distinct completed act. However, the nature of sin and of the perfection of God make this not just a matter of human acts in this manner. Sin, like salvation, is internal. One does not become hideous in form when one sins nor does one become worldly happy when one is a state of grace.
Not that which goeth into the mouth defileth a man: but what cometh out of the mouth, this defileth a man.
The internal aspect is what is most important. We know anything can happen to our flesh. Our inability to complete certain acts does not mean we are safe from the sins associated with those acts. While it is good for the community for murders to be uncommitted, it does one no good to hold within oneself the intent and desire to murder, yet not act merely because of social, physical, or mental constraints. Moral acts are not done because they are convenient, but because of a love of God, of what is good, which is often started with a basic fear of the wrath of God (which while imperfect is the beginning of perfection and thus enough for forgiveness in the sacrament of penance).
You have heard that it was said to them of old: Thou shalt not kill. And whosoever shall kill shall be in danger of the judgment. But I say to you, that whosoever is angry with his brother, shall be in danger of the judgment. And whosoever shall say to his brother, Raca, shall be in danger of the council. And whosoever shall say, Thou Fool, shall be in danger of hell fire.
This verse also shows the importance of understanding the intent and meaning of language. "Thou shalt not kill" is to be understood properly, not in an arbitrarily literal way (which gives it manner interpretations, but it is certain that the Law, and Our Lord in citing it, had a specific meaning which was commonly understood). But, as further expounded by Our Lord, the inner intent and will is more important than just the external act. This is the more common danger to us. We are unlikely to murder because of the great difficulty in mentally preparing for that act, the difficulty in committing the act, the great cost (in all aspects) of committing it in society, and other factors. Clearly, the average human in personal interaction is not inclined to murder. However, that does not mean we are naturally set on the path to God. Being unwilling or unable to commit the act because of such things does not mean one is good.
And again, the statements of Our Lord are to be understood as they were intended. The anger and words cited are not just literal, but a specific type of anger with a will and intent. There are no magically sinful words, which would be ludicrous, although it is actually a real mistake now where people of certain sects refrain from using the word "Father" in reference to their fathers, and condemn others for using the word, especially those in the Church who often refer to priests as father, because of a particular English translation of a scriptural verse about not calling any man our father. While this error is probably a result of a general intent to impugn the Church, rather than a misunderstanding of the verse (these people do not fret about "killing" in the most general sense, even though the law says "Thou shalt not kill", which would include all living matter), it does show that people can mentally choose the ridiculous to justify their position.
We should be careful not to commit a similar error, where our minds try to justify our acts and wills to make them "technically" alright. It is not a matter of a series of rules (except of course in certain disciplines established by those with authority, such as the bishops, our parents, and other people with lawful authority, but even these are restricted by a higher law), but of an internal cooperation with and acceptance of God and a reception of graces offered by God so we may shed our imperfections and become holy.
And this topic can lead to certain errors, especially for some people who suffer from particular afflictions, so keep in mind that all spiritual practices should be approved by one's spiritual director, especially if they are particularly "drastic" for one's state in life. Sanctifying Grace, the grace we receive in the sacrament of Baptism instituted by Our Lord, and can be obtained even with imperfection contrition (fear of punishment, pain, or sorrow) in the sacrament of Penance after sin, is only removed from us through mortal sin. Spiritual imperfection does not remove it, nor does lack of physical or spiritual consolation. Steadfastness is our calling. If we cannot feel good in body, mind, or spirit, or we cannot think we are good at all, it does not matter if we have chosen to cooperate with God and strive to do what we are called to do to the extent of not committing any mortal sin and following the precepts of the Church in the manner we our bound.