Keltham has probably had Peranza scried by now, he probably knows. Even if he could have forgiven everything else, he'll never forgive that, nor should he. It's - she has an intuition for Keltham - it's unforgiveable in a way most of the rest is only very very difficult to forgive. People should not end up worse off because they tried to help him. People ending up eternally tortured because they chose Civilization, tried to defect to it -
- he'll burn down all of Hell just for that.
And she could have said that to Abrogail. Could have told him that for that reason they should not do it. And then it would not have been done.