A bad penny always turns up.

 If "a bad penny always turns up", a bad person always

Give a thief enough rope and he'll hang himself. 

If "Give a thief enough rope and he'll hang himself" is true, what will happen to a bad person who is not challenged? In the end, he will get himself

Never do evil that good may come of it. 

If the saying "Never do evil that good may come of it" is right, then

The greater the sinner, the greater the saint. 

The saying "The greater the sinner, the greater the saint" implies that bad people

Two wrongs do not make a right. 

If "two wrongs do not make a right", we should counter one injustice with

A bad penny always turns up. If "a bad penny always turns up", a bad person always