Let me start by saying that God is full of unfailing love and compassion.
"God is not human, that he should lie, not a human being, that he should change his mind." Numbers 23:19
The coming of Jesus Christ(the new covenant) was prophesied in the old testament long before. In the old testament, people followed the law of Moses which gave them authority to judge(with unfairness or fairness)
You must not mistreat any widow or orphan.Exodus 22:22
They mistreated them anyway.You can't judge God for what those human judges did.Jesus exposed how hypocritical those judges were.Judge those human judges not God.
When they continued to question Him, He straightened up and said to them, “Let him who is without sin among you be the first to cast a stone at her.John 8:7
But in the new covenant, people live by faith and God is the only judge(He is always fair)
Why Jesus taught differently on “sin and treating sinners” as compared to Old Testament?
Jesus didn't teach differently on sin and treating sinners as compared to the old testament. God wanted people to repent in the old testament no matter how wicked they were.
“Go to the great city of Nineveh and preach against it, because its wickedness has come up before me.” Jonah 1.
" When God saw what they did and how they turned from their evil ways, he relented and did not bring on them the destruction he had threatened" Jonah 3.
Imagine being so wicked that God has to intervene.
Do you see how caring and loving He is? People usually can't show this type of forgiveness. I could do evil things before your eyes and even if I asked you to forgive me it would still take some times for you to forget and trust me, but God has the capability to forgive and forget your sins no matter how evil you are. Do you know how many prophets, king Ahab persecuted and killed? A lot, yet God forgave him. If someone could come and hurt all your relatives, would you forgive them and forget about it? Those prophets were God's children, yet our LORD forgave him.
Then the word of the LORD came to Elijah the Tishbite, saying: “Have you seen how Ahab has humbled himself before Me? Because he has humbled himself before Me, I will not bring the calamity during his days, but I will bring it upon his house in the days of his son.” 1 King 21:29
Wicked Sodom was a city full of rapists, yet God promised Abraham that He would spare the city if ten righteous were found. Would you spare a city full of terrorists(in the thousands)for the sake of 10 people?
He answered, “For the sake of ten, I will not destroy it.”Genesis 18:32
You said that Jesus was lenient towards sin compared to God(God never change)in the old testament but He really isn't. He want us to repent(just like the people of Nineveh) and He doesn't want us to perish
(but is patient with you, not wanting anyone to perish but everyone to
come to repentance. 2 Peter 3:9)
Jesus and the Father are one so there isn't a difference between them.
"I and the Father are one.” John 10:30
Sins and their wages according to the Messiah, who is God over all, forever praised! Amen
The Son of Man will send out His angels, and they will weed out of His kingdom every cause of sin and all who practice lawlessness. And they will throw them into the fiery furnace, where there will be weeping and gnashing of teeth.Matthew 13:41
Then He will say to those on His left, ‘Depart from Me, you who are cursed, into the eternal fire prepared for the devil and his angels. For I was hungry and you gave Me nothing to eat, I was thirsty and you gave Me nothing to drink,Matthew 25:41
But I tell you that anyone who is angry with a brother or sister will be subject to judgment. Again, anyone who says to a brother or sister, ‘Raca’ is answerable to the court. And anyone who says, ‘You fool!’ will be in danger of the fire of hell.Matthew 5:22
Being angry toward your brother is subject to judgement, calling someone a fool is reckless; how is it leniency? Jesus introduced new commands that are sometimes stricter than the old testament.
But I tell you that anyone who looks at a woman to lust after her has already committed adultery with her in his heart.If your right eye causes you to sin, gouge it out and throw it away. It is better for you to lose one part of your body than for your whole body to be thrown into hell.Matthew 5:29
But if you do not forgive men their trespasses, neither will your Father forgive yours. Matthew 6:15
Lusting toward someone wasn't a sin in the old testament and people used to divorce for any reason, unforgiveness was rampant also. Jesus made it more strict. Why would you Jesus tell us to gouge our eyes out if sin is not a big deal(aka leniency)
As regards to sin, God really never changes, see how loving and righteous He really is.
"Then He told them, “You are from below; I am from above. You are of this world; I am not of this world. That is why I told you that you would die in your sins. For unless you believe that I am He, you will die in your sins.” John 8:24
Many times the Pharisee tried to kill Him, yet He warned them time and time again so they could repent and believe in Him. But they couldn't, they had hearts of stone.
Good and upright is the LORD; therefore He shows sinners the way.He guides the humble in what is right and teaches them His way.Psalm 25:8
Old Testament we come across much harsher punishment for wrongdoing than what it is now.
God always wanted mercy. Sodom was full of rapists and homosexuals, yet they could have been spared if they had 10 decent people or didn't try to rape the angels.
For I desire mercy, not sacrifice, and the knowledge of God rather
than burnt offerings. Hosea 6:6
What can God do toward people who have no knowledge of Him and who keep doing what is wrong? God is Judge(Good and Upright,He is), He can't let chaos go rampant.
If you were a judge in a city, would you let crimes go rampant? Would you let go of a serial killer? As a judge, would you babysit a serial killer who can't stop killing people or a thief who can't stop stealing? (I'm using simple examples, I'm not trying to explain God, for His Ways are higher than my ways and His Thoughts are higher than my thoughts)
God shows mercy.
I will have mercy on whom I have mercy, and I will have compassion on whom I have compassion. Exodus 33:19
He always wanted people to repent
Do I take any pleasure in the death of the wicked? declares the Lord GOD. Wouldn’t I prefer that he turn from his ways and live? Ezekiel
Let the wicked man forsake his own way and the unrighteous man his own
thoughts; let him return to the LORD, that He may have compassion, and
to our God, for He will freely pardon. Isaiah 55:7
Return to Him against whom you have so blatantly rebelled, O children
of Israel. Isaiah 31:6
When God saw what they did, how they turned from their evil way, God
relented of the disaster that he had said he would do to them, and he
did not do it. Jonah 3:10
We are not to question His judgement
But who are you, O man, to talk back to God? Shall what is formed say
to Him who formed it, “Why have you made me like this?” Does not the
potter have the right to make from the same lump of clay one vessel
for special occasions and another for common use? Romans 9:20
Does an ax raise itself above the one who swings it? Does a saw boast
over him who saws with it? It would be like a staff waving the one who
lifts it! It would be like a staff lifting him who is not wood!
For He is not a man like me, that I can answer Him, that we can take
each other to court. Job 9:32