"People are not born into sin. People are born with an inclination to evil and may choose between good or evil. Again, this is clearly stated in Torah.
This is re-enforced in Genesis 4:7 when G-d speaks to Cain and tells him that good and evil are his choices and that he (Cain) should choose good. G-d tells Cain that he can master evil.
Ergo if Cain can CHOOSE he isnt born evil or born into sin."
The text actually says:
"Genesis 4:6 Then the Lord said to Cain, “Why are you angry? Why is your face downcast? 7 If you do what is right, will you not be accepted? But if you do not do what is right, sin is crouching at your door; it desires to have you, but you must rule over it.”
8 Now Cain said to his brother Abel, “Let’s go out to the field.”[d] While they were in the field, Cain attacked his brother Abel and killed him."
While Cain was presented with a choice, he refused to take the right road. Barnes in his commentary has an interesting take on this text:
"And if thou do not well, at the door is sin lying. - Sin past, in its unrequited and unacknowledged guilt; sin present, in its dark and stubborn passion and despair; but, above all, sin future, as the growing habit of a soul that persists in an evil temper, and therefore must add iniquity unto iniquity, is awaiting thee at the door, as a crouching slave the bidding of his master. As one lie borrows an endless train of others to keep up a vain appearance of consistency, so one sin if not repented of and forsaken involves the dire necessity of plunging deeper and deeper into the gulf of depravity and retribution. This dread warning to Cain, expressed in the mildest and plainest terms, is a standing lesson written for the learning of all mankind. Let him who is in the wrong retract at once, and return to God with humble acknowledgment of his own guilt, and unreserved submission to the mercy of his Maker; for to him who perseveres in sin there can be no hope or help. Another sentence is added to give intensity to the warning." (http://biblehub.com/commentaries/genesis/4-6.htm)
The only theory I have is Cain was like Abel at some point in his life. Either he was righteous like Abel by God's grace and he fell away, or he feigned goodness (Abel didn't feign goodness to clarify). But here is the thing, Sin is not so easy to master as many would like us to believe, but that is no excuse to deliberately submit to it and that is indeed what Cain, sadly did. For that matter, Cain would only be able to overcome his sin if he allowed God to do a mighty work in him to restore him but obviously Cain hardened his heart and refused.
Anthony Rogers of Answering Islam provided his comments on this issue when I inquired about this point:
""People are born with an inclination to evil..."
True, people are inclined toward evil from birth. But this isn't the way God made man, i.e. Adam, and is a result of the fall.
In addition, this inclination to evil, being contrary to man's original "goodness," i.e. contrary to what man was when he first came from the hand of his Maker, is itself evil. Notice, for example, that Genesis 6 does not simply say that man is inclined to evil but that his inclinations themselves are evil:
"5 The Lord saw that the wickedness of man was great in the earth, and that every intention of the thoughts of his heart was only evil continually. 6 And the Lord regretted that he had made man on the earth, and it grieved him to his heart. 7 So the Lord said, “I will blot out man whom I have created from the face of the land, man and animals and creeping things and birds of the heavens, for I am sorry that I have made them.” 8 But Noah found favor in the eyes of the Lord."
What distinguished Noah and set him apart from the people of his time was not that he was good and chose the good but that he found FAVOR/GRACE in the eyes of the Lord.
Genesis 4:7 does say that sin is crouching at the door and that Cain must master it, but it does not say that he had the ability to do so, at least not of himself. The only way Cain would have been able to overcome his own (sinful) bent toward sin is if Cain, like Noah, would have "found favor in the eyes of the Lord.""
Hopefully the comments have been addressed adequately.