After twenty-eight letters, Eva doesn't seem any closer to figuring out her son than she was when he was born. But she has decided to accept him, for better or for worse.
Okay, there is no better. It's all for worse.
Even though we know Kevin's crime from the beginning (or from reading the back of the book), we don't know right away how he did it, and Eva lays out every gruesome detail so that she—and we—can be aghast at how terrible the crime is. We're not sure what is more shocking, the violence of the crime, or the diabolically perfect way Kevin carried it out.
One thing Eva finally does that she hasn't been able to do in the years after Kevin's crime is to ask him flat-out why he did it. He thought he knew. "Now I'm not so sure," he says (28.52).
Eva seems to have given up trying to find any answers… because there aren't any. As a result, she says she is "too exhausted and too confused and too lonely to keep fighting, and if only out of desperation or even laziness, I love my son" (28.73). Both she and Kevin are insanely stubborn, and both seem to have given in, just a little bit.
It's not a happy ending, but what other ending can there be?