God did not create man with an evil nature, He created us as children, innocent and not knowing the good from evil, this knowledge was gained when Adam and Eve ate from the Tree of Knowledge. By partaking of the fruit, it allowed Adam and Eve to know what is good and bad, what is virtue and vise, and what is pleasure and pain, which is why, in Genesis 3 of the King James version of the Bible it says,
6 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, she took of the fruit thereof, and did eeat, and gave also unto her husband with her; and he did eat.
7 And the eyes of them both were opened, and they knew that they were naked; and they sewed fig leaves together, and made themselves aprons.
8 And they heard the voice of the Lord God walking in the garden in the cool of the day: and Adam and his wife hid themselves from the presence of the Lord God amongst the trees of the garden.
9 And the Lord God called unto Adam, and said unto him, Where art thou?
10 And he said, I heard thy voice in the garden, and I was afraid, because I was naked; and I hid myself.
Children do not understand that nudity is a bad thing - just as Adam and Eve did not understand - many young children may take off their clothes anywhere and in front of anyone. But as the child grows older they come to a "knowledge" that nudity is wrong.
After God found that Adam and Eve were no longer childlike he created enmity (animosity, hatred) between Satan and the children of Adam and Eve so that we would understand and strive to choose the right. God also sent Jesus Christ to be our Savior and our example of perfection so that through our faith in Him we may return to the presence of our Father. (See John 3:16)
16 ¶For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.
God's intention was never to punish us, no parent enjoys disciplining their child, but for a child to learn, there needs to be consequences. This same relationship is true between ourselves and our Heavenly Father. The punishment for Adam and Eve was that they were banned from the Garden of Eden, otherwise they would have eaten from the Tree of Life and lived forever in their wickedness.
As for your question about becoming sinless, in Genesis 3 it says:
22 ¶And the Lord God said, Behold, the man is become as one of us, to know good and evil: and now, lest he put forth his hand, and take also of the tree of life, and eat, and live for ever:
It is not that our freewill is taken away from us, it is that we have a perfect understanding of good and evil. We are able to choose because we will know the consequences of such actions and we will choose not to sin, because "no unclean thing can dwell in the presence of God."