Has God really changed?
Well, the Bible is very clear that God never changes (Malachi 3:6) and that He is the same yesterday, today and forever (Hebrews 13:8). God is completely sovereign and perfect, so there is no reason for Him to change His ways because a perfect God will only have perfect actions. When you read passages in the OT that talks of God ending the lives of people, God always has a reason for that and all the time His reason is because of the wickedness of those people. Psalm 19:9 tells us that God's judgment is true and righteous altogether.
When it comes to God's patience with people, that has never changed. In fact, His patience and mercy for people has been showcased even in the OT, as evidenced by this verse from Lamentations 3:22 that says, "It is of the Lord's mercies that we are not consumed, because his compassions fail not." In Ezekiel 18:23, it is clear that God finds no pleasure in the death of the wicked and the reason to this is because He does not find pleasure in seeing people going to Hell for their sins. This was again voiced in the NT in 2 Peter 3:9 when it says that "God is not willing that any should perish but that all should come to repentance."
How about God's hatred of sin? It seems that God hates sin more in the OT but is more passive in the NT in dealing with it. That is so untrue! God's hatred for sin in the OT is the same hatred he had in the NT. Remember the couple in the book of Acts that God killed because they lied or the king in the same book that he killed while delivering a speech because he glorified not God but instead became proud of himself? I don't see there God becoming more allowing of sin, but instead, I see Him as being consistent from who He was in the OT.
And if you say that God was more gracious and merciful of sinners in the NT than in the OT as proven by how He gave Himself a ransom for sin when He died on the cross, you have failed to see the other side of the coin in that instance. Yes, Jesus' sacrifice at the cross was a show of love, a show of grace for sinners like you and I. But you have to also see God's hatred of sin there. In His hatred of sin and in His demand for justice, He willingly poured out all of His wrath on His one and only Son so that justice is served. Now if that is not hatred of sin, I really don't know anymore what you can call that. And mind you! The history of this world is not yet over. One day, God will finally put an end to sin and give every sinner his/her deserved punishment in Hell.
Now this is the grace of God. He is giving you two choices. Its either you let Jesus pay for your sins by His death on the cross through your repentance and faith on Him, or you pay your sins, yourself, by going to Hell for an eternity of unthinkable punishment either when your life ends soon or you live long enough to witness God coming back to earth at the end of history to get the justice He demands. You make the choice.