How does killing bear consequences? How can I explain to a non follower of the dhamma that killing living beings intentionally has mental and physical consequences?

This person started saying: "old hunter-gatherers, had no consequences nor problems with hunting". If I tell them this is the Buddhist point of view they are going to label it as religious mystical nonsense.
How can I explain to a non follower of the dhamma that killing living beings intentionally has mental and physical consequences?


Kamma is intention. The intention dictates the mental and physical consequences. 


  This person started saying: "old hunter-gatherers, had no consequences nor problems with hunting". 


Correct. They hunted for food; to feed their children; following the cycle of life; how nature created them. 


  If I tell them this is the Buddhist point of view they are going to label it as religious mystical nonsense.


The above appears to be your personal point of view rather than a Buddhist point of view. If the above idea was Buddhist, there would be specific quotes from scriptures to support this idea; where the Buddha instructed people to not kill for food. Obviously, in the Buddha's time, people would have asked him the same questions about how to gather food. Obviously, the later monks decided such questions did not warrant inclusion in the scriptures. We know the Buddha never taught vegetarianism because he allowed monks to eat meat from almsfood. The Buddha appeared concerned about killing with wanton violence (rather than about how people feed themselves).