God has to obey his justice.

God’s justice requires that sin be punished:  so It is not that God is without mercy, but he is bound by his words.

The bible tells us in Romans 6:23 , ” for the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord”.

(Sin has affected the human race so much that taking another man’s life becomes easy. God on the other hand wants to give eternal life to all who would be obedient to his words.