Did God's Personality Change from Old to New Testament?

The simple answer is, no. However, I realize that as you read the OT and see the attacks the Lord orchestrates against certain people groups it does not seem consistent with Jesus' declaration about the blessing that comes to peacemakers, for instance.
We know that Jesus is the exact representation of the Father. Jesus told Philip that since he had seen Him, he had seen the Father. When God declares His Name in Exodus 34:6, 7, He says He is compasssionate and gracious, slow to anger and abounding in lovingkindness and truth who keeps lovingkindness for thousands, who forgives iniquity, transgression and sin; yet He will by no means leave the guilty unpunished, visiting the iniquity of fathers on children and on the grandchildren to the third and fourth generations.
God is compassionate and full, or abounding in, lovingkindness. We know from the OT that He "delights in mercy". The last thing He mentions in the declaration of His Name is basically that He is pefectly just. The guilty cannot go unpunished. I was reading in Leviticus, today,and was just noticing all the things people were to be put to death for doing. There were a lot of situations where violation of that law was punished by death. Several of the situations, such as adultery, you can see how God would show no tolerance and this was a great deterrent to an expanding adultery problem. In contrast, our society today shows relatively little outrage, horror or even much concern over adultery. If you didn't notice it is spreading like wild fire. A little leaven leavens the whole lump of dough. When the source is stopped so is the problem. In the OT several societies were destroyed on command from God because to allow them to continue on and curse the next generations with their sin was a worse alternative than to eliminate the source of the problem in that generation.
However, even in that, God was not without mercy, or hasty to bring punishment. Before Israel was allowed to go in and take their land, first the cup of iniquity of the Amorites had to be full. God waited for them, but there was no repentance. Jonah preached a message of judgment upon Nineveh. We are not told in the message that he even mentioned repentance, but when the city repented God had mercy.
We actually see the same thing taking place at the end of the age. When the sixth trumpet is blown there are horrendous calamities, but we read in Revelation 9:20, 21 that people will not repent. When people no longer repent God unleashes the seven bowls of wrath which are loosed when the seventh trumpet is sounded. Why? Because God is not willing that any should perish but all should come to repentance. He waits to see if there is any repentance left before the final judgment is released. This is consistent with His actions OT or NT. When there is no longer any repentance there is nothing left , but wrath. This is OT and NT alike.
Since God is just He cannot, by His nature, just say, that's o.k., no big deal, you're forgiven. Every sin must be dealt with according to the rightful payment. It is with this in mind that we can somehow deal with Isaiah's prophecy about the Father's hand against his own Son when it says, "But the Lord was pleased to crush Him, putting Him to grief; If He would render Himself as a guilt offering." It "pleased" the Lord to crush him. What it did, was satisfy the necessary just payment, so that, in view of justice being rendered mercy could then be offered. Justice was dealt out, but wait, Who was it dealt upon? He himself provided the sacrifice. He paid for us. The sermon on the mount lifestyle is following this same pattern. Bless those who curse you, pray for your enemies is taking "the hit" ourselves, rather than returning eye for eye and tooth for tooth. Eye for eye and tooth for tooth is perfectly just. God took justice for us upon Himself and now leads us in a lifestyle that follows that pattern. He is abounding in "lovingkindness", or mercy. There comes a time where repentance is no longer going to come that it is better to end the source of the problem than to allow it to spread upon forthcoming generations. That is in God's hand to decide.
He brutally dealt with our sin problem at the cross. Our sin was not without consequence. It is now that by grace, He took the consequence Himself. God was pleased to satisfy the just requirements of the Law upon Jesus so that we could go free. Our sin was brutally dealt with just as OT sin was brutally dealt with. We were just fortunate to receive the payment on our behalf by faith.