These are my opinions
Why would God harden pharaohs heart so he would not allow the Israelites to leave, but that later say that He will destroy the Egyptians because he would not allow them to leave? What the reason for that? To say/ show that He is Lord?
I think Paul gives us understanding of how and why God "hardened Pharaoh's heart":[VERSE=Romans 1:18-22 and 28-31, RSV]For the wrath of God is revealed from heaven against all ungodliness and wickedness of men who by their wickedness suppress the truth. For what can be known about God is plain to them, because God has shown it to them. Ever since the creation of the world his invisible nature, namely, his eternal power and deity, has been clearly perceived in the things that have been made. So they are without excuse; for although they knew God they did not honor him as God or give thanks to him, but they became futile in their thinking and their senseless minds were darkened. Claiming to be wise, they became fools. And since they did not see fit to acknowledge God, God gave them up to a base mind and to improper conduct. They were filled with all manner of wickedness, evil, covetousness, malice. Full of envy, murder, strife, deceit, malignity, they are gossips,slanderers, haters of God, insolent, haughty, boastful, inventors of evil, disobedient to parents, foolish, faithless, heartless, ruthless.[/VERSE]I ran across this explanation about God's hardening the hear of Pharaoh in a commentary written by Gregory of Nyssa in the 4th century:
Who is it who is delivered up to shameful affections can be clearly learned from the Apostle: It is he who does not like to have God in his knowledge. God delivers up to passion him whom he does not protect because he is not acknowledged by him. But his failure to acknowledge God becomes the reason why he is being pulled down into the passionate and dishonorable life (The Life of Moses)
Also - in Genesis, God said He would never again destroy the people from the earth, but then destroyed the Egyptians.. why?
It's surprising that your version would translate it that way. I found the following, based on all the various Hebrew texts that have been around:[VERSE=Genesis 8:21, Septuagint - Based on oldest known Hebrew texts - around 150 BC]I will not any more curse the earth, because of the works of men, because the imagination of man is intently bent upon evil things from his youth, I will not therefore any more smite all riving flesh as I have done.[/VERSE][VERSE=Genesis 8:21, Latin Vulgate - Based on Hebrew texts available in 4th century AD]I will no more curse the earth for the sake of man: for the imagination and thought of man’s heart are prone to evil from his youth: therefore I will no more destroy every living soul as I have done.[/VERSE][VERSE=Genesis 8:21, Syriac Peshitta - Based on unknown Hebrew text - perhaps around the 1st century AD]I will not again curse the ground any more for the man's sake: for the inclination of man’s heart is evil from his youth: neither will I again destroy any more every thing as I have done.[/VERSE][VERSE=Genesis 8:21, Masoretic Text - Based on Hebrew texts available sometime after 600 AD]Never again will I doom the earth because of man, since the devisings of man's mind are evil from his youth; nor will I ever again destroy every living being, as I have done[/VERSE]Based on all these, I think the promise was not to destroy
all men, but this doesn't mean that God promised never to destroy certain individuals or groups throughout history (e.g. Sodom and Gomorrah, Genesis 19).
