Primarily irreligious cultures still do not, as a rule, tolerate murder.
Why?
Probably has something to do with people seeing all human life as innately valuable, or they understand that if murder is tolerated, then they themselves may be murdered, and they don't want to die.
And which cultures specifically are you talking about?
All of them, pretty much! I'm not a sociologist but I'm not actually aware of any society which is particularly tolerant of murder; by all means, provide a counterexample. Japan and Sweden work as two examples of clearly less religious countries no more tolerant of murder than any other.
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