Heinrich Himmler explained it precisely in his speech at Posen, October 1943 delivered to the SS in which he spoke of the extermination of the Jewish people:

"We had the moral right, we had the duty to our own people, to kill these people who wanted to kill us. We have carried out this most difficult task out of love for our own people. And we have suffered no harm to our inner self, our soul, our character in so doing."

Koenigsberg explains: In this statement, Himmler provides the foundation for a theory covering all forms of collective violence: war, genocide and terrorism.

(1) In begins with the assumption that the other group wishes to kill or severely harm one's own group (members of one's own group).

(2) This being the case, the leader or leaders of the group about to be killed, harmed or victimized believe they have the moral right, indeed a duty to kill those people who want to kill or harm members of one's own group. Acts of war, genocidal violence, and acts of terror are carried out (however difficult) in the name of love for one's people.

(3) Leaders and nations who carry out these violent acts suffer no harm to their soul or character--because these acts are carried out in the name of love for one's people; in the name of the survival of one's own people.
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Himmler's speech: complete text