Thank you for your question
1. Is God a fair or a just God?
There are several factors to consider.
God is the creator, we are his creation and as such we have no right to demand either fairness or justice from God.
Romans 9:20 But indeed, O man, who are you to reply against God? Will the thing formed say to him who formed it, "Why have you made me like this?"
21 Does not the potter have power over the clay, from the same lump to make one vessel for honour and another for dishonour?
Fortunately for us our creator is both fair and just.
The conversation recorded between Abraham and God over the fate of Sodom is a good illustration.
Genesis 18: 23 And Abraham drew near, and said, Wilt thou also destroy the righteous with the wicked?
24 Peradventure there be fifty righteous within the city: wilt thou also destroy and not spare the place for the fifty righteous that are therein?
25 That be far from thee to do after this manner, to slay the righteous with the wicked: and that the righteous should be as the wicked, that be far from thee: Shall not the Judge of all the earth do right?
26 And the LORD said, If I find in Sodom fifty righteous within the city, then I will spare all the place for their sakes.
27 And Abraham answered and said, Behold now, I have taken upon me to speak unto the Lord, which am but dust and ashes:
28 Peradventure there shall lack five of the fifty righteous: wilt thou destroy all the city for lack of five? And he said, If I find there forty and five, I will not destroy it.
29 And he spake unto him yet again, and said, Peradventure there shall be forty found there. And he said, I will not do it for forty’s sake.
And on to verse 32
32 Then he said, "Let not the Lord be angry, and I will speak but once more: Suppose ten should be found there?" And He said, "I will not destroy it for the sake of ten."
There were only four righteous found in Sodom – Lot and his wife and two daughters and so Sodom perished.
2 Peter 2:6 And turning the cities of Sodom and Gomorrha into ashes condemned them with an overthrow, making them an ensample unto those that after should live ungodly;
7 And delivered just Lot, vexed with the filthy conversation of the wicked:
8 (For that righteous man dwelling among them, in seeing and hearing, vexed his righteous soul from day to day with their unlawful deeds;)
9 The Lord knoweth how to deliver the godly out of temptations, and to reserve the unjust unto the day of judgment to be punished:
The Character of God is declared to Moses at Sinai.
Exodus 34:6 And the LORD passed by before him, and proclaimed, The LORD, The LORD God, merciful and gracious, longsuffering, and abundant in goodness and truth,
7 Keeping mercy for thousands, forgiving iniquity and transgression and sin, and that will by no means clear the guilty; visiting the iniquity of the fathers upon the children, and upon the children’s children, unto the third and to the fourth generation (of them that hate me –Deuteronomy 5:9).
We see that God is not only fair and just but also merciful, prepared to forgive the wrongdoing of thousands.
The only standard that man has of fairness and Justice is that of God.
The Law given through Moses has no equal in the laws devised by men.
Man is incapable of devising a standard of fairness and Justice.
A check on the judicial systems of this world will justify the well-known saying ‘the law is an ass’ (first coined by Charles Dickens in Oliver Twist).
Solomon with his God-given wisdom says:
Ecclesiastes 7:20 For there is not a just man upon earth, that doeth good, and sinneth not.
Experience teaches us the truth of this. We all fall short of the righteousness required by God.
We are unqualified by our very nature to decide ultimately what is fair and just.
Jeremiah 17:9 The heart is deceitful above all things, and desperately wicked: who can know it?
The sentence for sin is death. Once we sin we do not have a right to a long life.
Ezekiel 18:4 Behold, all souls are mine; as the soul of the father, so also the soul of the son is mine: the soul that sinneth, it shall die.
20 The soul that sinneth, it shall die. The son shall not bear the iniquity of the father, neither shall the father bear the iniquity of the son: the righteousness of the righteous shall be upon him, and the wickedness of the wicked shall be upon him.
The reason that we do not die immediately we sin is because God gives us time to repent.
Ezekiel 33:11 Say unto them, As I live, saith the Lord GOD, I have no pleasure in the death of the wicked; but that the wicked turn from his way and live: turn ye, turn ye from your evil ways; for why will ye die, O house of Israel?
2 Peter 3:9 The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to us–ward, not willing that any should perish, but that all should come to repentance.
In the prophecy given by Jesus to John we are told that the justness of God will be applauded when his purpose comes to fruition.
Revelation 15:3 They sing the song of Moses, the servant of God, and the song of the Lamb, saying: "Great and marvellous are Your works, Lord God Almighty! Just and true are Your ways, O King of the saints!
The fairness and justness of God can be seen in the gospel records of the life of Christ.
Christ was the manifestation of God’s character in man.
Luke 9:56 For the Son of man is not come to destroy men‘s lives, but to save them.
God has appointed Christ to be the future judge.
Acts 17:31 Because he hath appointed a day, in the which he will judge the world in righteousness by that man whom he hath ordained; whereof he hath given assurance unto all men, in that he hath raised him from the dead.
John 5:22 For the Father judgeth no man, but hath committed all judgment unto the Son:
This in itself is a just decision because Christ experienced for himself the nature that we struggle with.
Hebrews 4:15 For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin.
After his resurrection he ascended to heaven to become our mediator.
1 Timothy 2:5 For there is one God, and one mediator between God and men, the man Christ Jesus;
1 John 2:1 My little children, these things write I unto you, that ye sin not. And if any man sin, we have an advocate with the Father, Jesus Christ the righteous:
1 John 1:9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
This seems to me to be eminently fair and just.
2. Why do you think god killed all those people but not Noah and his wife and his three sons and their wives?
The reason is clearly given.
Genesis 6:12 And God looked upon the earth, and, behold, it was corrupt; for all flesh had corrupted his way upon the earth.
13 And God said unto Noah, The end of all flesh is come before me; for the earth is filled with violence through them; and, behold, I will destroy them with the earth.
God is a God of love and he expects his creation to love one another.
Even when mankind had reached the point of filling the earth with violence he still gave them another 120 years.
Genesis 6:3 And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
Noah was a just man and for this reason he found favour in God’s sight.
Genesis 6:8 But Noah found grace in the eyes of the LORD.
9 ... Noah was a just man and perfect in his generations, and Noah walked with God.
During the 120 years Noah built his ark as a witness to them and preached to the people - all to no avail.
The other thing to remember is that this life is in reality a time of proving for the eternal life that is promised to them that live by the commandments of God.
For those who are accounted worthy to attain unto the resurrection of the dead to eternal life it does not matter whether this mortal life is long or short.
Luke 20:34 And Jesus answered and said to them, "The sons of this age marry and are given in marriage.
35 "But those who are counted worthy to attain that age, and the resurrection from the dead, neither marry nor are given in marriage;
36 "nor can they die anymore, for they are equal to the angels and are sons of God, being sons of the resurrection.
Like awakening from a night’s sleep the next thing they will know after death will be the resurrection to eternal life.
I hope this helps