Heidelberg Catechism Lesson 40

Q. 105. What doth God require in the sixth commandment?
 
That neither in thoughts, nor words, nor gestures, much less in deeds, I dishonour, hate, wound, or kill my neighbour, by myself or by another;[1] but that I lay aside all desire of revenge:[2] also, that I hurt not myself, nor wilfully expose myself to any danger.[3] Wherefore also the magistrate is armed with the sword, to prevent murder.[4]
[1] Matthew 5:21–22; Proverbs 12:18; Matthew 26:52;  [2] Ephesians 4:26; Romans 12:19; Matthew 5:39–40;   [3] Matthew 4:5–7; Colossians 2:23;  [4] Genesis 9:6; Matthew 26:52; Romans 13:4.
 
 
Q. 106. But this commandment seems only to speak of murder?
 
In forbidding murder, God teaches us, that He abhors the causes thereof, such as envy, hatred, anger, and desire of revenge;[1] and that He accounts all these as murder.[2]
[1] James 1:20; Galatians 5:20; Romans 1:29; 1 John 2:9;  [2] 1 John 3:15.
 
 
Q. 107. But is it enough that we do not kill any man in the manner mentioned above?
 
No: for when God forbids envy, hatred, and anger, He commands us to love our neighbour as ourselves;[1] to show patience, peace, meekness, mercy, and all kindness, towards him,[2] and prevent his hurt as much as in us lies;[3] and that we do good, even to our enemies.[4]
[1] Matthew 22:39 and 7:12;   [2] Romans 12:10; Ephesians 4:2; Galatians 6:1–2; Matthew 5:5; Romans 12:18; Exodus 23:5;  [3] Matthew 5:45;  [4] Romans 12:20.

Commentary

The Sixth Commandment, “Thou shalt not kill,” is perhaps the most well known of the Ten Commandments. Many modern professors of faith, who are unable to cite most of the commandments, and even unbelievers, will be quick to cite this commandment when asked what is the Ten Commandments. Most people know it is wrong to commit murder. Yet, ironically, this is the command (together with the Eighth Commandment) that many people in the world will use to show that they are good and righteous. They say, “We are good people. See, we do not kill nor steal, and therefore we can expect to go to heaven when we die.” We see from such a statement that many regard murder as the greatest sin, and assume that if they do not commit murder, that they are therefore innocent.
 
Not only does the world refuses to believe that a failure to worship and honour the living and true God as God (First Commandment) is in fact the greatest sin vis-à-vis the Ten Commandments, but the world forgets that the Sixth Commandment is not only about physical murder. This is what the Q/A’s above are seeking to address. Any deed or thought that is unloving towards another is in fact a breaking of the Sixth Commandment.
 
The Lord Jesus, we must remember, is He who most clearly show us that this is the right way to interpret the Sixth Commandment and all other commandments. He says:

Ye have heard that it was said by them of old time, Thou shalt not kill; and whosoever shall kill shall be in danger of the judgment: But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire (Mt 5:21–22).

By these words, the Lord teaches us that we must not merely equate the Sixth Commandment with murder, but with any act that may be in anyway related to murder. Thus unrighteous wrath, vengeance, malicious thought, slander, suicide, failure to ensure safety and health, etc., are all transgressions of the Sixth Commandment. In fact, a failure to love our neighbour as ourselves is failing to meet up to the requirements of the Sixth Commandment.