“Good” and “bad” are relative terms that lead to much subjectivity. “Fit” and “unfit” can have some measurable criteria to which you can deem a person “fit” or “unfit”.

I don’t think you can equate fit to good and unfit to bad. How would you define good and bad? Would someone who has a mental infirmity be a ‘bad’ person?

Just because someone is unfit to do something doesn’t mean that they are bad person. It’s similar to someone who is fit to drive versus unfit to drive. It doesn’t make the person bad. It does make them however unfit to drive any vehicle as they pose a threat to themselves or a threat to others.