Adam and Eve were more(?) righteous because they had faith.

Genesis 15:6 (NASB)
Then he believed in the LORD; and He reckoned it to him as righteousness.

If "partial credit" righteousness could be calculated, it would be calculated (in a practical sense—since no sinners are righteous and they were all sinners, Ro 3:10) by belief/trust/faith in God.

If it means anything to compare the righteousness of Adam/Eve to that of Cain, then Adam and Even were "more" righteous than Cain only because they were righteous and Cain was not (Jude 1:11, 1 Jo 3:12).

Adam, Eve and Cain were all guilty of the same sin; they all misplaced their faith. Adam and Eve placed their faith in the words of the serpent rather than the Word of God:

Genesis 3:1-6 Now the serpent was more subtle than any beast of the field which the LORD God had made. And he said unto the woman, Yea, hath God said, Ye shall not eat of every tree of the garden…And when the woman saw that the tree was good for food, and that it was pleasant to the eyes, and a tree to be desired to make one wise

Cain placed his faith in himself:

Hebrews 11:4 By faith Abel offered unto God a more excellent sacrifice than Cain

Genesis 4:3 Cain brought of the fruit of the ground an offering unto the LORD…But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell.

Cain’s countenance fell because he expected God to receive his offering. If Cain had received by faith his offering from the ground (as God insists) and God rejected it, he would have been confused not angry. Cain failed to receive by faith the fruit of the ground. God is never impressed with our independent initiatives, but only His initiatives through our life.

Adam and Eve wanted to determine for themselves what was good and evil, it should be no surprise that their offspring wanted to justify himself before God. All three placed their faith outside of God’s love, God’s grace.

Answer: (a) Cain was a worse sinner than Adam/Eve.

Why?

(1) Cain was a murderer but Adam and Eve were not.

(2) Cain had no love for God while Adam and Ever had love for God and they even guided their children how to worship God. For instance, they told Cain and Abel to give offerings to God. Cain was not giving his best offering to God or he was not offering from his heart, as a result God rejected his offering.

(3) Cain did not show respect to God:

Then the Lord said to Cain, “Where is your brother Abel?” “I don’t know,” he replied. “Am I my brother’s keeper?” (Genesis 4:9, NIV)

(4) Adam and Eve had faith in God: Though Eve was the cause of failure, she knew her sins and she still had faith in God. When Eve gave birth to Cain, she gave thanks to God and acknowledged Him.

Adam made love to his wife Eve, and she became pregnant and gave birth to Cain. She said, “With the help of the Lord I have brought forth a man.” (Genesis 4:1, NIV)

After Abel was murdered by Cain, Cain also ran away, leaving his parents alone. However, Eve again gave birth to Seth and gave thanks to God for providing her a substitute to Abel.

Adam made love to his wife again, and she gave birth to a son and named him Seth, saying, “God has granted me another child in place of Abel, since Cain killed him.” (Genesis 4:25, NIV)

My first response is that there is little value in debating which of two people are the "worse sinner". We are all sinners who need to be saved by grace. Whether a rapist is worse than a murderer, etc, is not a conversation that accomplishes much.

But okay, I think almost any Bible student would say that the CONSEQUENCES of Adam and Eve's sin was worse than the consequences of Cain's sin. Romans 5:12, "Therefore, just as through one man sin entered the world, and death through sin, and thus death spread to all men, because all sinned ...", and 5:18a, "Therefore, as through one man’s offense judgment came to all men, resulting in condemnation ..." Also 1 Cor 15:22, "For as in Adam all die ..." Adam's sin brought sin and judgment on all humans. No such thing is said of Cain.

On the other hand, in Genesis 4 Adam and Eve talk about being blessed by God, which would seem to indicate that they repented from their sin, while Cain does not appear to have repented. It's not spelled out and of course it's not for me to judge, but I THINK that in the end Adam and Eve were saved while Cain was not.

We might also note that God does not hold us accountable for the consequences of our actions so much as for our intentions. Jesus said that if you hate your brother that's no different than murder. I think what he meant was that if person A wants to kill someone and does it, while person B wants to kill someone but is unable to follow through because of circumstances -- perhaps his intended victim is too well guarded -- that in God's eyes both are equally guilty. C.S. Lewis wrote that one man may be in a position where when he gets angry, he can order the deaths of thousands of people, while another man may be in a position where when he gets angry, people just laugh at him, but God judges both the same.

I don't know where you get the part about Adam and Eve supposing that they were more righteous than their children. The Bible doesn't say one way or the other, whether they were egotistical and self-righteous or humble.