If we believe that God intended to lead Israel into the Promised Land and that He was going to be the One to drive out the Canaanites, and if you acknowledge that instead Israel had to fight (although blessed by God), then you need to acknowledge that something changed. However, to get a complete picture of what changed and why, we would need to go back to the beginning. We need to look at the first murder and how the perpetrator of that murder was treated in relation to those who came later.
Consider that in both murder and in war, someone dies. True, it is possible in theory to have a “war” without someone dying, but then it would probably be called something else. Therefore, murder is a decent parallel to point out how God has treated humanity over time.
However, it is worthy to note that there is a legal distinction between murder and capital punishment, no matter how much spin and baloney you put on it. Legally, murder is unjustified and punished, while capital punishment is often the punishment dealt out for murder. The OT Law also contains this provision. If someone was guilty of murder, he or she was often stoned to death. The exception was an accident, for which the killer was allowed to live but restricted to a city of refuge.
Therefore, another way you could put it is that capital punishment was ordained by God as a just punishment for certain crimes while murder covers the taking of another human life under circumstances God decrees as a violation of His Law. In short, capital punishment is sanctioned while murder is not. That distinction will play into this much later.
However, what does the Law say?
6At the mouth of two witnesses, or three witnesses, shall he that is worthy of death be put to death; but at the mouth of one witness he shall not be put to death. (Deuteronomy 17:6, King James Version)
And yet, what happened in Cain’s case? Did Adam and Eve hold a trial of their son? Were Cain’s brothers and sisters (he obviously at least had one sister) called in as witnesses?
No, God Himself confronted Cain.
There are several valid possibilities as to why God confronted Cain, but the truth is that we aren’t told why. However, God did confront Cain instead of holding a trial as outlined in the Mosaic Law.
However, I would be remiss if I did not point out that it wasn’t the Mosaic Law that required a civil government to punish murderers. It was much, much earlier. In fact, it is the very first command we see that even gives a hint of a civil government.
But, we’ll save that for next time.