“Mercy killing” (intentionally ending a person’s life to relieve suffering) is not presented in the Bible as something God commands or approves.

What the Bible does clearly emphasize instead is:

Do not murder: Scripture repeatedly treats the intentional killing of an innocent human as wrong (e.g., “You shall not murder” in the Ten Commandments).

Human life has divine value: People are portrayed as having worth because they’re made by God (e.g., Genesis 1:26–27). That theme pushes against the idea that some lives can be ended because they’re suffering.

Care for the suffering: The Bible strongly supports relieving suffering—through compassion, medical help, and communal care—without framing “ending the sufferer’s life” as an acceptable solution.

The Bible contains both strong moral constraints against killing and narratives of people who used force, including some cases where God or Israel’s leaders sanctioned violence. There isn’t a single verse that cleanly “permits self-defense killing,” but you can use the Bible’s themes to form an answer.

What the Bible clearly affirms

Human life has deep moral weight. “You shall not murder” (common OT command; echoed in the NT as the moral heart of the command).
Justice is not the same as revenge. The Bible repeatedly pushes restraint, due process, and leaving vengeance/judgment to God rather than personal retaliation.
Roughly: “don’t kill, and don’t become the aggressor.” Even when force is used in the Bible, it’s typically framed as defense/justice, not bloodlust.
Where self-defense fits (the closest “textual pattern”)

1) “Don’t murder,” not “never use force.”
The Bible’s command targets murder—an unjust killing. That distinction is important because self-defense is usually argued as “unjust killing is what you must not do,” while defensive force aims to stop an immediate threat.

2) Cases that look like legitimate defense/necessity

Property/household protection logic: OT law includes situations where stopping an intruder could be lawful (the details differ by case), which shows the principle that protection of life/property can matter under certain conditions.
City-of-refuge/judicial restraint: The legal system treats some killings as accidental/justifiable vs. intentional—again implying the Bible distinguishes motives and circumstances.
What this means for “killing in self-defense”

From a biblical-theology perspective, the safest synthesis is:

Self-defense is morally framed as preventing harm, not escalating violence.
It should be limited and proportionate to the threat (stop the attacker, not punish them beyond the threat).
It should avoid retaliation once the danger has passed.
It should be handled within justice frameworks as much as possible (reporting, courts, accountability), not private vengeance.
Key ethical cautions people often miss

The Bible repeatedly condemns anger-driven killing, revenge, and blood-guilt.
Even if a person “survives,” the Bible’s moral emphasis pushes toward taking responsibility, not treating lethal force as a right to act at will.
If you want, tell me the tradition/framework you mean (e.g., Catholic, Protestant evangelical, Orthodox, Jewish reading, or “just the Bible text”) and what scenario you have in mind (immediate physical attack? weapon involved? can you escape?), and I’ll map the biblical principles to that specific case.

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In the book of Judith, Judith is portrayed as righteous, not as a sinner, and the text explicitly frames what she did as an act of deliverance under God’s power.

1) The narrative treats her killing as God-enabled deliverance
Judith is single-mindedly seeking to “destroy the enemies who have risen up against us” and she prays that God would “look at this hour upon the work of my hands” for the glory of Jerusalem (Judith 13:4–5). When she decapitates Holofernes, the story quickly shifts to worship and blessing rather than condemnation.

2) Judith herself denies wrongdoing and highlights purity/motivations
Afterwards she testifies that she “deceived him with my appearance in order to kill him,” but adds: “he committed no sin with me, to pollute or shame me” (Judith 13:16). That line matters because the story is concerned with her moral integrity—what would have “polluted” her—not with her condemning the enemy by force.

3) The leaders and people bless her
When she returns, the community responds with praise and blessing, saying God “destroyed our enemies by my hand” (Judith 13:15) and affirming she was protected and guided. Uzziah even calls her “blessed” and credits God with guiding her “to cut off the head of our enemy’s leader” (Judith 13:18–20).

So: has she sinned or been righteous?

Within the book’s own moral logic: Judith is righteous. Her act is presented as God’s instrument to save Israel, not as transgression.

If you want, I can also explain how different Jewish/Christian traditions interpret the book overall (some read it as theological/moral narrative rather than literal history, which affects how people talk about “sin” vs “righteousness” in her action).

“Be not forgetful to entertain strangers: for thereby some have entertained angels unawares.”
‭‭Hebrews‬ ‭13‬:‭2‬ ‭KJV‬‬

O Lord our Heavenly Father, high and mighty King of kings, and Lord of lords, who dost from thy throne behold all the dwellers on earth and reignest with power supreme and uncontrolled over all the Kingdoms, Empires and Governments; look down in mercy, we beseech Thee, on these our American States, who have fled to Thee from the rod of the oppressor and thrown themselves on Thy gracious protection, desiring to be henceforth dependent only on Thee. To Thee have they appealed for the righteousness of their cause; to Thee do they now look up for that countenance and support, which Thou alone canst give. Take them, therefore, Heavenly Father, under Thy nurturing care; give them wisdom in Council and valor in the field; defeat the malicious designs of our cruel adversaries; convince them of the unrighteousness of their Cause and if they persist in their sanguinary purposes, of own unerring justice, sounding in their hearts, constrain them to drop the weapons of war from their unnerved hands in the day of battle!

Be Thou present, O God of wisdom, and direct the councils of this honorable assembly; enable them to settle things on the best and surest foundation. That the scene of blood may be speedily closed; that order, harmony and peace may be effectually restored, and truth and justice, religion and piety, prevail and flourish amongst the people. Preserve the health of their bodies and vigor of their minds; shower down on them and the millions they here represent, such temporal blessings as Thou seest expedient for them in this world and crown them with everlasting glory in the world to come. All this we ask in the name and through the merits of Jesus Christ, Thy Son and our Savior.

Amen.