Laudator Temporis Acti

Aesop, Fables 209 Perry, 313 Chambry, 225 Hausrath  (tr. Robin Waterfield):

A shepherd found some wolf cubs and brought them up with a great deal of care because he thought that when they were adult they would not only protect his own sheep but also steal other people's sheep and bring them to him. But as soon as they were grown up, the first thing they did was take advantage of the shepherd's carelessness and kill his sheep. And the shepherd groaned and said, "It serves me right! Why did I save their lives when they were little, since I've got to kill them now when they're big?"

Likewise, those who keep bad people safe are failing to realize that all they're doing is empowering them to do them harm.

Ποιμὴν εὑρὼν λυκίδια, ἔτρεφεν ἐπιμελῶς, οἰόμενος ὅτι μεγαλυνθέντα τηρήσουσι τὰ ἑαυτοῦ πρόβατα, ἀλλὰ καὶ ἔτι προσθήσουσιν ἁρπάζοντες ἕτερα καὶ εἰσάξουσιν ἐν τῇ αὐτοῦ μάνδρᾳ. Οἱ δὲ, ὡς ηὐξήνθησαν, πρῶτον αὐτοῦ τὴν ποίμνην διέφθειραν. Καὶ ὃς ἀναστενάξας εἶπεν· Δίκαια πέπονθα· τί φὰρ μὴ νηπίους ἀπέκτεινον;

Ὁ μῦθος δηλοῖ ὅτι οἱ τοὺς πονηροὺς διασώζοντες λανθάνουσι καθ' ἑαυτῶν πρῶτον αὐτοὺς ῥωννύντες.