T'is said everyone has value even if it's only to serve as a BAD example. Where is the value in that? What do we gain, for how long and why?

Those who are useful only as BAD examples have committed crimes or otherwise treated people despicably and done horrendous things. Wouldn't we better off without them completely? How many BAD examples must we endure to understand BAD? I don't get it. I don't agree with it. Everyone having value is debatable in my opinion.  What is your and why?