They look the same to me. I do not understand the distinction you tried to make. The Old Testament insists on the death penalty and makes no bones about it. This is true of your Numbers and Genesis quotations. The New Testament, especially the Gospels, urges mercy and restraint.
I want to respect your Christianity; if there is a difference between the Old and New Testaments I would have thought the Words of Christ in the Gospels over-ruled Genesis and Numbers. I do not share your faith yet I still prefer the Gospels' humane stance to the barbarism of ancient nomadic tribes with the blood lust expressed in those OT quotations.
Okay, here's something else then. You say you see a difference between the Old and New Testaments. In the Old Testament, the Lord condones the death of a murderer while in the New Testament, He tells us not to take vengeance for our wrong, correct?
In Matthew 5:17-18 it says "Do not think that I have come to abolish the law or the prophets. I have not come to abolish these things, but to fulfill them. I tell you the truth, until heaven and earth pass away not the smallest letter or stroke of a letter will pass from the law until everything takes place."
Genesis 9:6, "Whoever sheds human blood, by other humans must his blood be shed; for in God's image God has made humankind."
Hebrews 13:8, "Jesus Christ is the same yesterday, today and forever."
Exodus 21:12-14, "Whoever strikes someone so that he dies must surely be put to death. But if he does not do it with premeditation, but it happens by accident, then I will appoint for you a place where he may flee. But if a man willfully attacks his neighbor to kill him cunningly, you will take him even from my altar that he may die."
These are a few verses concerning both God's Word on murder and His unchanging ways.
Job 34:10-12, "Therefore, listen to me, you men of understanding. Far be it from God to do wickedness, from the Almighty to do evil. For He repays a person for his work, and according to the conduct of a person, He causes the consequences to find Him. Indeed, in truth, God does not act wickedly, and the Almighty does not pervert justice."
Acts 5:1-11, "Now a man named Ananias, together with Sapphira his wife, sold a piece of property. He kept back for himself part of the proceeds with his wife’s knowledge; he brought only part of it and placed it at the apostles’ feet. But Peter said, “Ananias, why has Satan filled your heart to lie to the Holy Spirit and keep back for yourself part of the proceeds from the sale of the land? Before it was sold, did it not belong to you? And when it was sold, was the money not at your disposal? How have you thought up this deed in your heart? You have not lied to people but to God!” When Ananias heard these words he collapsed and died, and great fear gripped all who heard about it. So the young men came, wrapped him up, carried him out, and buried him. After an interval of about three hours, his wife came in, but she did not know what had happened. Peter said to her, “Tell me, were the two of you paid this amount for the land?” Sapphira said, “Yes, that much.” Peter then told her, “Why have you agreed together to test the Spirit of the Lord? Look! The feet of those who have buried your husband are at the door, and they will carry you out!” At once she collapsed at his feet and died. So when the young men came in, they found her dead, and they carried her out and buried her beside her husband. Great fear gripped the whole church and all who heard about these things.
Deuteronomy 32:4, "As for the Rock, His work is perfect, for all His ways are just. He is a reliable God who is never unjust, He is fair and upright."
These verses explain how God is righteous in all His ways, how He is a just God, punishing every wrongdoing perfectly, even if that's not what it looks like to us.
These last verses will be concerning God's love and His constancy.
James 1:17, "All generous giving and every perfect gift is from above, coming down from the Father of lights, with whom there is no variation or the slightest hint of change."
Psalm 86:15, "But you, o Lord, are a compassionate and merciful God. You are patient and demonstrate great loyal love and faithfulness."
Hebrews 6:17-18, "In the same way, God wanted to demonstrate more clearly to the heirs of the promise that His purpose was unchangeable, and so He intervened with an oath, so that we who have found refuge in Him may find strong encouragement to hold fast to the hope set before us through two unchangeable things, since it is impossible for God to lie."
Deuteronomy 7:9-10, "So realize that the Lord your God is the true God, the faithful God who keeps covenant faithfully with those who love Him and keep His commandments, to a thousand generations, but who pays back those who hate Him as they deserve and destroys them. He will not ignore those who hate Him but will repay them as they deserve!"
Numbers 23:19, "God is not a man, that He should lie, nor a human being, that He should change His mind. Has He said, and will He not do it? Or has He spoken and will He not make it happen?"
I know this is a long, long post, but each verse is important and carefully selected. This is why I believe there is no difference between the teachings of the Old and the New Testaments. They may look different, but as the Bible clearly teaches, the Lord does not go back on His word. And He has declared that for the one who sheds blood, who willfully kills, they shall and must be killed.