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.