Understand that Adam and Eve were the most human of humans. God created them Himself, and so they had the purest nature that a human could have. This would mean that it was natural for the people to give in to temptation and curiosity at least once in infinity. And because there is no need of the concept of time in heaven, one could say that the one-in-infinity chance was taken.
It proves that men have weaknesses and can choose to resist evil, but could fall too. What happened, had to happen.
Now, for Cain and Abel. There are slightly different traditions in different faiths about the story.
God punished the murder by wasting his work of soil, and if God explicitly punished it, then the murderer must have repented.
In the details that Islam gives about the story, Cain repented right after killing his brother. He then saw a bird that buried a dead bird in the ground, and felt ashamed of himself to be so foolish as to leave his brother's body on open ground, and buried him. Repentance is not found in heaven so when Adam was first cast to earth, he felt something that he did not understand. God then taught him words of repentance so that he understands his crime and seeks forgiveness.
Adam is also the first messenger of God on earth, he would not be punished for being human.
