God destroyed the world with a flood, destroyed all the people in Sodom and Gomorrah, gruesome sacrifices, laws for stoning people, Jericho, requesting the sacrifice of Abraham's son, the sacrifice of God's own son Jesus'. Jesus seems so different to me because instead of making life or taking life he gave his own, he preached forgiveness and grace and seemed to defy the laws made in Leviticus. I'm a christian but I am having a hard time understanding this, especially if one believes the Trinity doctrine.
If you look at the role of Jesus especially at the very end of time and the beginning of eternity (eternity for us, that is to say) he does not seem that different after all. When Jesus returns, he will not return as the contemporary hippie Jesus. That image is a product of modern humanistic culture and not the Jesus of scriptures. He will return as the King of kings, terrifying in his righteous wrath. That will be a day of devastation for anybody who does not want anything to do with him.
One striking difference between Noah and the flood vs Abraham and Sodom/Gomorrah is that Abraham was the one who thought that perhaps there was still hope for the two cities. Noah did not. Abraham pleaded with God and God agreed upon a minimum number of righteous people who, if present, would be grounds for not carrying out the punishment. A similar incident happened during the early exodus, when God, appalled at the golden calf the Israelites had made, offered Moses to destroy them all and make him the new god's people. Moses turned the offer down.
It is almost as if God tries to provoke a reaction. If Moses' reaction had been "yes, let's do away with them wrongdoers" God most certainly would have carried out his threat, as his righteousness demands. God never bluffs. At the same time He would have been disappointed in the hard-heartedness of Moses.
I believe this type of thinking can be applied to the mosaic law. It gives a human being an apparent right to stone another, but it does not take away the possibility to show compassion. The problem is that the israelites rarely did that. It tells more about man than it tells about God. The tendency to point fingers at each other and not see our own error is a product of the god-syndrome mankind fell victim of in the garden of Eden. That is something Jesus pointed out, and that is just what he did; he did not introduce a new hippie religion or change, defy, or undo the law, but pointed out how the law should have been applied in the first place. He explained what the law really is and is supposed to be; a dilemma.
I hope this answer helps and manages to touch all the different issues in your question.
I speak only for myself here.
If you read through the OT quickly, you will "see the forest for the trees", that is, you will get an overarching picture of who God is and what His purpose for us is. This will often get lost if you focus only on the details of the OT.
Many scholars have discussed the way God interacted with an ancient Near Eastern people, and there are good sources* to read about that. There are some who don't believe in the genocides, the flood, etc.
Even if these narratives are true, the OT teaches us about the Holiness of God, His lovingkindness in the face of our repeated faithlessness, His allowing us free will and to reap the consequences of our sins, and our need for an intercessor - His son. At the appropriate moment in History, He sent us the intercessor, that was foretold in all of the OT. In essence, I don't think we could understand the magnitude of our separation from God to truly appreciate the sacrifice of His son without the OT.
Any attribute of Jesus can be found in the Father. Also, to underscore God's place, the Greatest Commandments are “'Love the Lord your God with all your heart and with all your soul and with all your mind.' This is the first and greatest commandment. And the second is like it: 'Love your neighbor as yourself.' All the Law and the Prophets hang on these two commandments.” Jesus does not, ever, contradict any part or action of His father.
*Inspiration and Incarnation: Evangelicals and the Problem of the Old Testament, by Peter Enns
There is not difference. Jesus, like all the other OT prophets, preached that judgement was on its way. Though he did offer forgiveness, it came with an "or else."
Matthew 24:37-39 King James Version (KJV)
37 But as the days of Noah were, so shall also the coming of the Son of man be.
38 For as in the days that were before the flood they were eating and drinking, marrying and giving in marriage, until the day that Noe entered into the ark,
39 And knew not until the flood came, and took them all away; so shall also the coming of the Son of man be.
2 Thessalonians 1:8-9 King James Version (KJV)
8 In flaming fire taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ:
9 Who shall be punished with everlasting destruction from the presence of the Lord, and from the glory of his power;
Many other judgement verses in the NT could be cited, such as most of the book of Revelation.
Both God and His Son are consubstantial - of same nature.
Hebrews 1:3 (NASB)
And He is the radiance of His glory and the exact representation of His nature, and upholds all things by the word of His power. When He had made purification of sins, He sat down at the right hand of the Majesty on high,
John 10:28-30 (NASB)
28 and I give eternal life to them, and they will never perish; and no one will snatch them out of My hand. 29My Father, who has given them to Me, is greater than all; and no one is able to snatch them out of the Father’s hand. 30 I and the Father are one.”
Both God the Father and God the Son have the same wrath.
Revelation 6:16-17 (NASB)
16and they said to the mountains and to the rocks, “Fall on us and hide us from the presence of Him who sits on the throne, and from the wrath of the Lamb; 17for the great day of their wrath has come, and who is able to stand?”
What is different from God and Jesus is not their equality of nature but rather, their function or role.
That there is a difference between the portrayal of God in the Old Testament and the portrayal in the New Testament is clear, hence the question. This led Marcion, in the early second century, to decide that the Supreme God who sent Jesus could not have been the Creator God of the Old Testament. In the Antithesis, he said
The Creator God is judicial, harsh, and mighty in war. The Supreme God is gentle and simply good and excellent.
The Creator God is inconsistent, in respect of persons, sometimes disapproving where approbation is deserved; or else lacking in foresight, bestowing approbation on men who ought rather be reprobated, as if he either censured his own past judgements, or could not forecast his future ones.
Jesus Christ and none else revealed a new God, who, in the Old world and in the Old time and under the Old God was unknown and unheard of...
Marcion might have been wrong in his concept of a Creator God (Old Testament) and a Supreme God (New Testament), but he did clearly see the difference in the two accounts.
If, as modern Christianity certainly teaches, there are not two different Gods, then the difference may be explained as a cultural one. The authors of the Hebrew Bible believed in a harsh and judgemental God, so this is how they most often wrote of him. Jesus, and the gospel authors who gave us his words, believed in a loving, forgiving God, and so this is how the Father is portrayed throughout the New Testament.
Marcion's understanding of two Gods is incompatible with the Nicene doctrine of the Holy Trinity, and the modern contrast is between the Old Testament concept of God and Jesus.