Pirkei Avot 2:14

"He said to them: Go out and discern which is the evil path from which a man should distance himself.  Rabbi Eliezer says? An evil eye.  Rabbi Yehoshua says: A wicked friend.  Rabbi Yose says: A wicked neighbor.  Rabbi Shimon says: One who borrows and does not repay; one who borrows from man is like one who borrows from the Omnipresent, as it is said, 'The wicked one borrows and does not repay, but the Righteous One is gracious and gives' (Psalms 37:21).  Rabbi Elazar said: A wicked heart.  He [Rabban Yochanan be Zakkai] said to them: I prefer the words of Elazar be Arach to your words for your words are included in his words."

A wicked friend, a wicked neighbor and one who borrows but does not repay are all other people.  Other people can certainly have an influence on a person, but if a person has strong morals, he can resist their influence.  But why is a wicked heart more to be avoided that a wicked eye?  A wicked eye is a self centered way of viewing the world, seeing the actions of others as motivated by greed or jealousy, and being inclined to be motivated exclusively by one's self interest. Such a way of viewing the world is certainly likely to result in self-centered actions.  However the heart, or the inclination to kindness or unkindness, is the more fundamental source of character, because it influences the person's way of viewing the world itself.  If one has a good heart, he is more likely to view the actions of others in a charitable light, and be motivated to act kindly toward others because he has an appreciation of kindness.  The inclination, be it good or evil, tends to lead the view of the world in the same direction.