the Sovereign One

I can't accept this.
God can forgive. Can't he?
Why did the Israelite stone Achan?
Why did men kill men?

Yes, God has His standard.

Light and dark cannot coexist. Neither can black and white. Grey is not a darker version of white. It is simply not white.
When God said to Joshua, "Stand up! What are you doing down on your face? Israel has sinned; they have violated my covenant, which I commanded them to keep. They have taken some of the devoted things; they have stolen, they have lied, they have put them with their own possessions. That is why the Israelites cannot stand against their enemies; they turn their backs and run because they have been made liable to destruction. I will not be with you anymore unless you destroy whatever among you is devoted to destruction."

That is God's standard.

Men are in fear of the Lord back then. The relationship between men and God are rigid and fearsome. 
Do you think that it is possible that men made the decision to kill Achan? Although God allowed it, but it was not God's intention.
Look at Moses, he pleaded for the Israelite several times and God listened to him. I think God would have listened to anyone if they has pleaded on behalf of Achan. But the thing is no one did.
Men held many assumptions of God. God allowed them because it is within the boundary of His allowance for free will. Overall, we need to know that God is in control even when life seems out of control.