Let literal language be literal and figurative language be figurative. And watch out for idioms, which have special meanings.
Rule 1, definition of "evil" and "eye" - no help here. Rule 2, context: seems to confuse us even more. It doesn't seem to fit with what goes before and after! This should tip us off that we aren't understanding it rightly!!
What we have here is a Hebrew idiom, “evil eye”. Let's look up other uses of this idiom: Mt.20:15 " Is it not lawful for me to do what I wish with what is my own? Or is your eye envious [lit."evil"] because I am generous [lit. "good"]?" We find that having an "evil eye" is a Hebrew idiom for being stingy or envious. Now go back to Mt.6 and notice how this understanding ties in so perfectly to the context.
Is.59:1 “The Lord's hand is not short;”
Deut.33:27 “Underneath are the everlasting arms.”
References to body parts of God are used by Latter-Day Saints to prove that God was once a man just as we are. Once they convince people of that, they go on to teach that we can become God just like He is! At a lecture he was giving, a group of Mormon elders challenged Walter Martin (author of Kingdom of the Cults) with an enumeration of verses like these. Dr. Martin then asked the Mormons to read one more scripture: Ps.91:4 “He will cover you with His feathers; And under His wings shalt thou trust”. W.M. said, “By the same rules of interpretation that you just proved God to be a man, you just proved that He is a bird”. The Mormons had to laugh as they realised the ridiculousness of their position.