This is the only verse from which I find personal spiritual instruction in a chapter detailing how to deal with murder and manslaughter. God is making a point to Israel: senseless, non-defensive taking of a human life disrespects HIS holiness. He would not tolerate it.
That principle holds true for today because it is rooted in relationship with God. That exists now, though in a different dimension than that which Israel experienced as a theocracy. Taking of a human life defiles what God desires. And Israel was to obey God in judicial handling of human justice in these matters, which really is the whole point of Numbers 35.
So the principle is simple but impacting: human life is treated as special and important out of reverence for God. Remove any reverence for God, Who is truly holy, and human uniqueness devalues as well. Voila! We have today. In a truly secular society there is little basis of human life being valued. And today their reigns a growing disrespect for human life all the way from conception to the last breath in old age.