Lots of people seem to forget Andy arguably is a murderer
Some people think Tommy's story is a lie, and Andy really did kill his wife and her lover, but that's not the point of this post, let's assume Tommy's story is true, and Andy really didn't kill them.
It depends how you define the word "murderer". If you define it as someone who has committed murder, Andy isn't a murderer. If you define it as someone who has murderous intentions, Andy is a murderer. He actually went to the lover's house, and stood outside the door with a loaded gun, with the intention of breaking in and killing them, before he changed his mind. That in itself is a crime punishable by jail time.
You could argue this makes him a bad person. There are some situations where even committing murder doesn't make you a bad person, such as Samuel L Jackson's character in A Time To Kill, who killed the men who raped his daughter. But Andy just wanted revenge for an affair, and later he admitted he drove her away. That doesn't make cheating OK, but to almost commit murder over it does kinda make him a bad person.