The Bible doesn't state it is an evil tree. God said in Genesis 2:17,"But of the tree of knowledge of good and evil, thou shalt not eat of it…" Its name indicates that eating its fruit would in some way open the eater's eyes to good and evil. And that is exactly what happened, "… (Eve) did eat, and gave also unto her husband with her; and he did eat. And the eyes of them both were opened." Genesis 2:6, 7 Later, when God talked with Adam and Eve, He said, "Behold, the man is become as one of us, to know good and evil…" It seems reasonable that their act of willful disobedience to God by eating the fruit was what opened their eyes to evil.
Why did God create the tree? The Scriptures do not specifically explain why, but the fact Adam was commanded not to eat it would seem to indicate this was a test of obedience to God. Would Adam value his relationship to his God of love, justice, wisdom, and power? Or would Adam choose to follow his own will? Adam chose to oppose God's will. Consequently, he (and the entire human race of his children) has reaped the consequences of sin and death.