Excerpts from here:
Sentence of death is allowed only through the process of justice, but even then, forgiveness is better. Nor take life – which Allah has made sacred – except for just cause (17:33).
Before the advent of Islam, in the Arabian Peninsula, retaliation was a common phenomenon. If someone was killed, the victim’s tribe would take action against the entire tribe of the killer. This monstrous attitude was forbidden through the Quran. The Quran admonishes those who oppress others and transgress the bounds of what is right and just. The blame is only against those who oppress men with wrongdoing and insolently transgress beyond bounds through the land, defying right and justice. For such there will be a chastisement grievous (in the Hereafter) (42:42).
Harming innocent bystanders, even in times of war, was forbidden by Rasulullah(S). This includes women, children, non-combatant bystanders, and even trees and crops. Nothing is to be harmed unless the person or thing is actively engaged in an assault against Muslims. ye who believe! (Do not) kill yourselves, for truly Allah has been to you Most Merciful. ..(4:29-30).
“If anyone slays a person – unless it is for murder or for spreading mischief in the land – it would be as if he slew all people. And if anyone saves a life, it would be as if he saved the life of all people.” (5:32)
Goodness and evil are not equal. Repel evil with what is better. Then that person, with whom there was hatred, may become your intimate friend! And no one will be granted such goodness except those who exercise patience and self-restraint, none but people of the greatest good fortune. (Surat No. 41: Ayaat Nos. 34-35).