Why did God said not to murder but there's David glorified for killing Goliath and many other stories like so

i want a Christian point of view. no others. i was talking to my friend about Christianity and that's what he's caught up on. well that and ALOT of other stuff. but one thing at a time right? okay. so God says don't murder. but he gives people the strength to kill. helps people win battles.... show more i want a Christian point of view. no others.

i was talking to my friend about Christianity and that's what he's caught up on.
well that and ALOT of other stuff.
but one thing at a time right?


okay. so God says don't murder. but he gives people the strength to kill. helps people win battles. kills families.

please explain.

i've looked everywhere. ):
Update: what happened to "turning the other cheek"
Update 2: my bad for the funny looking question.
i had to shorten it from "how come" to make it fit.
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