Source: Gaijin Pot
The two characters for good and evil (or bad), 良（よ）い and 悪（わる） respectfully, are really useful characters.
Give us the bad news first
The 悪 kanji, for example, can also be added as an adjective to the front of many words to mean an unpleasant version of that.
Take, for example, 悪口（わるぐち） which means “to talk badly of someone.” It can also be added as a suffix to make words like:
良い/いい, on the other hand, has the opposite effect and describes a particularly pleasant or nice example of that thing. Generally, いい is more common in spoken Japanese with the exception of 良いお年（とし）を which is another way to wish someone a Happy New Year.
The Japanese use いい a lot in their language.
So far, so simple, right? However, from here things get a little more interesting. Both 悪い and いい/ 良い have some fascinating grammatical usages that all learners should be familiar with.
Readers growing up in the 90s will recognize the use of (僕（ぼく）) が悪い as similar to that old stalwart of 90s-ness: the saying ‘”My bad’… presumably with a “dude” not far behind. While this may sound a little dated in English, it’s still a beloved expression among Japanese people.
This phrase is mostly used to admit a mistake, but it can also be used alongside certain disrespectful pronouns, as in sentences like お前（まえ）が悪いよ (You are wrong!) and だれが悪いのか (Who is at fault?).
～が悪い is also used in some other forms to create descriptions of things.