The reason why "good vs. evil" is a universal theme is because there are always two forces at work one for good and one for evil and for every good/evil thing there is an evil/good thing that will go up against each other until the end of time or at least until one of them is defeated, but then another good/evil thing will appear to take it place.
Example 1:
Your in high school and there is a bully and you try to stop the bully from hurting others. This makes you the good force and the bully the evil force (he/she doesn't have to be "evil" per-say just bad or mean will do).