Version One of "I'm no saint" means the person does not claim to be perfect, but strives to do what is right. The expression is an admission, but not a surrender or an excuse.
Version Two means the person has given up trying to do what is right unless doing so is convenient. These people use the expression as an excuse to continue unethical behavior. In extreme cases, they may have no interest whatsoever in doing what is right and may even take pleasure in doing wrong.
The people who follow Version Two can bring down an organization. Given time, they may also bring down a society.