The fifth commandment is “You shall not kill.” It commands us to take proper care of our own spiritual and bodily wellbeing and that of our neighbour. It forbids us to kill or injure ourselves or others in soul or body.

We have a serious obligation not to expose ourselves or others to grave dangers for soul or body unnecessarily. Grave dangers for soul are proximate occasions of mortal sin, such as viewing immoral materials and visiting immoral places. Examples of grave dangers for the body are reckless driving and abusing drugs.

Caring for the soul

We should not cause the spiritual injury or spiritual death of ourselves or others. The soul is spiritually injured by venial sin, and spiritually killed by mortal sin.

Bad example and scandal are sins against the wellbeing of the soul. Scandal is committed when any help or encouragement is given in any way to cause another to do wrong. If we have been the occasion of scandal, we are bound to repair the mischief done.

We should not give scandal and should avoid cooperating in the sin of others. There are nine ways of cooperating in sin: by counsel, by command, by consent, by provocation, by praise or flattery, by concealment, by being a partner in it, by silence, and by defending it.

Caring for the body

The fifth commandment forbids murder, suicide, fighting, anger, hatred, revenge, drunkenness, abusing drugs, reckless driving, and anything else hurtful to the body.

Direct killing of a person by another is always wrong, except in certain cases when the killing is not direct, or when it is done by public authority. There are three such cases.

The first case is self-defence, or the defence of another unjustly attacked. The person who is unjustly attacked should not intend the death of his assailant, but simply his own preservation.

One may not do more than what is needed for defence; if wounding the assailant is sufficient, it would be wrong to kill him. Also, a person is not justified in killing in order to protect property of small value.

The second case is executing criminals condemned by legitimate public authority. No private person or mob has the right to put anyone to death. Regarding the death penalty, the Catechism of the Catholic Church states:

“Assuming that the guilty party’s identity and responsibility have been fully determined, the traditional teaching of the Church does not exclude recourse to the death penalty, if this is the only possible way of effectively defending human lives against the unjust aggressor.”

“If, however, non-lethal means are sufficient to defend and protect people’s safety from the aggressor, authority will limit itself to such means, as these are more in keeping with the concrete conditions of the common good and more in conformity wilth the dignity of the human person.”

“Today, in fact, as a consequence of the possibilities which the state has for effectively preventing crime, by rendering one who has committed an offence incapable of doing harm without definitely taking away from him the possibility of redeeming himself the cases in which the execution of the offender is an absolute necessity ‘are very rare, if not practically nonexistent.'”

The third case is just war. Father John Hardon, SJ, has written, “A war is considered just when declared by proper authority and undertaken as a last resort, and when those waging war use no more destructive means than are necessary to achieve an early and just peace. The purpose for which the war is fought must be a good one.”

“For You have dominion over life and death” (Wis 16:13).

「天主十誡」中的第五誡是「毋殺人」，命令我們要適當地照顧自己和別人的靈魂與身體，禁止我們對兩者的傷害或殺戮。

我們有切實的本份，不把自己和別人的靈魂，置於靈性死亡—-犯大罪—的危險中；例如觀賞不道德的東西、瀏漣不道德場所等。我們也不應讓自己和別人的身體，置瀕險境；例如危險駕駛、濫用藥物等。

保守靈魂

我們都應避犯小罪，以免傷害自己和別人的靈魂，更不應自犯或誘人觸犯大罪，讓靈魂墮入永遠的喪亡。惡行或壞表樣，都歸納在這誡命嚴禁之內。一旦我們協助或慫恿他人的惡行，我們必須負上彌補的責任。

除了戒立惡表，我們對任何惡行，均避免共犯。九種情況搆成共犯之嫌，包括對惡行作出建議、指令、允許、挑釁、讚許或讚賞、隱瞞、參與、緘默和維護。

保守身體

第五誡禁止謀殺、自殺、毆鬥、忿怒、憎恨、報服、酗酒、濫藥、魯莽駕駛，和一切傷害身體的事。

基本上，私人直接地殺人必然是錯的，但在非直接殺人或合法公共職權下殺人的情況下，亦非絕對搆成罪行。有三種情況可以被接受殺人無罪。

假如一個人被襲擊，又或為保護貴重財物，在別無他法、無可選擇的情況下，可被接納為自衛而殺人。自衛者的意向只可以是保護自己，絕對不可是他人的死亡。如果打傷襲擊者便能把他制服，自衛者便不可殺害他。一個人絕對不可為了保護小許的財物而殺人。

執行死刑是合法殺人。當然，這是指大眾公認的合法裁決和合法執法而論，私刑或非大眾公認的合法裁決而執行死刑，不在此列。

天主教對於執行死刑，有如下指引（天主教教理2267）：

「假設有罪一方的身分和責任已完全被確定，教會的傳統訓導並不排除訴諸死刑，但只要這是唯一的可行之道，藉以有效地保護人命，免受不義侵犯者之害。」

「如果非殺傷性的方琺足以衛護人們的安全，免受侵犯者之害，掌權者只應採用這些方法，因為這些方法更符合公益的具體條件，也更合乎人性的尊嚴。」

「事實上，今日由於國家具有各種有效地防止犯案的可能性，使犯罪者不得再逞，而不至於決定性地剝奪其改過自新的機會，因此，絕對必須處決罪犯的個案就『十分罕見，即使並未完全絕跡』。」

公義的戰爭是可以接受。耶穌會會士若望.赫頓神父寫道：「合法的掌權者為了維護公義，如果戰爭是唯一方法，那可以為公義和善意的目的，發動戰爭；但這戰爭應盡量減低其傷害性和破壞性，並在最短時間內，達到公義和平而結束。」

「因為祢掌握生死的大權。（智慧篇16:13）」