In the history of Talentless Nana, the government did form a law-enforcing organization comprised of Talented individuals, but they eventually revolted, which explains why the students are being killed instead of getting proper training. Could the revolution have been caused by the Talented losing their morality? Perhaps. But then why was this information kept from Nana while she was still on the side of the government? There's no way this was a recent discovery, considering the Talented have been around for over a hundred years already.

This is my answer — Tsuruoka only came up with this lie to get Nanao on his side so he could get his hands on the Talent to nullify other Talents. Whatever that monster in containment was, it doesn't represent the Talented as a whole. It is just a rare worst case scenario. However, even without transforming into bloodthirsty monsters, the Talented have far more power than the Talentless; hence the government's plan to systematically eradicate them. As for Tsuruoka's own motives, I believe he is looking to gain control, and Nanao can grant him that over the Talented especially.

And if the author knows what's good for them, they will confirm Tsuruoka is lying.

regardless narratively the guy is lying but plot-wise it kinda slightly fits