Praise be to Allaah.
Killing a non-Muslim when he is a muaahid (one of those who have a peace treaty with the Muslims) is a sin, one of the major sins. Al-Bukhaari narrated that Abd-Allaah ibn Amr ibn al-Aas (may Allaah be pleased with them both) said: The Prophet (peace and blessings of Allaah be upon him) said: Whoever kills a muaahid will not smell the fragrance of Paradise, even though its fragrance may detected from a distance of forty days. But with regard to non-Muslims who are at war with the Muslims and do not have a peace treaty with the Muslims or are not living under Muslim rule, then Muslims are commanded to kill them, because Allaah says (interpretation of the meaning):
“Fight those of the disbelievers who are close to you, and let them find harshness in you” [al-Tawbah 9:123]
But this should be in the case of jihaad under the leadership of one of the leaders of the Muslims, or his deputy.
Shaykh Abd al-Kareem al-Khudayr . (www.islam-qa.com)
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