And he lived through the hard choice regarding Lizzie, the choice to take her out to prevent her from harming others. He didn't do it but he didn't stop Carol either. So when she said she killed the others to keep them from infecting the rest of the prison it was harder for him to be self righteous and see it all in black and white terms. And he saw how she suffers, just as he knows he'll suffer for what happened to the kids he was protecting and for his part in it. And so he decided it wasn't for him to judge her.
Am I the only one who thought Tyreese was going to pick up the gun and kill himself?