why is this quote significant to the meaning of the novel? #3

And perhaps you might pretend, afterwards, that it was only a trick and that you just said it to make them stop and didn’t really mean it. But that isn’t true. At the time when it happens you do mean it. You think there’s no other way of saving yourself and you’re quite ready to save yourself that way. You want it to happen to the other person. You don’t give a damn what they suffer. All you care about is yourself.