I don't want my friend to be my best man at my wedding anymore
What happened between you that you don't want your friend being best man?
I just think he'd be a bad best man.
If you let him know why and know someone else who can do it, I think it's alright to change who your best man is. I think a lot of people change their mind about different things that occur in a wedding.
Am I right or wrong in doing this?
You should realize that he may be hurt by it, but you want your wedding to go well. If you don't think he's up for the job, then you should feel okay replacing him. Make sure you explain it to him in a kind way though.