How about I tell my friend I won't be best man at his wedding a week before the date?
Why won't you be his best man there?
He has chosen a woman he loves and I do not wish to go to the wedding.
That is going to make the wedding very hard for him. You shouldn't back out if you agreed to do it and should just get your job done and be there for him.
I don't see why I should do it if I don't want to.
You gave your commitment to your friend to be his best man. If you back out a week before the wedding, you are going to cause major issues for him.