What do tu think of my theory?

If ketchup is red and mayo is white...their mix is pink!
We should call it pinkayochup!

That's called resisting temptation. The reason there's evil is because man gave into temptation.

Well, it isn't really your theory, per se. It's a general critical theory that applies to a pair of terms that is opposite in meaning. According to this theory one exists simply because the other does. It's called Binary Oppositions. If there exists evil there will be good to counter it, and vice versa. It's a concept of structuralism and fundamental human philosophy.