God believers kill too, they just get forgiveness afterward. If a non-religious person believes they shouldn't kill, they also won't kill. Religion doesn't prevent killing. Most of the world is religious, and killing happens all the time.
The point is murder. Religion have not much to do , the point is to obey the commandment , and if there is a commandment there is a giver of the commandment , the giver is an entity ( God ) . A believer believes God who see our action and know our intent . A secular does not have that cast, he can hide its action from others and can act at his will, if there are not other observers. ( Remember murder is something premeditated )
In our daily life as an example we break our traffics laws provided there is no police to pull us in.( this is a minor offence ) Murder is different
I grant you there are many secular people who are law obedient and respect life, perhap even more than some religious.