No contradiction.
The stranger who "forgave" Susan was seizing a moral high ground that she had done nothing to earn. She was playing the morally superior victim when she was not in the least the aggrieved party.
Susan is not responsible for her son's sins and she owed this person no apology.
A question, though. Where did all those guns come from ? Were they Klebold's or Harris's or both ? Now if they were Klebold's and she was sitting back doing nothing while her son was amassing enough firepower to outfit a squad, then she was grossly derelict of her duty.
The person might have been trying to communicate a collective I forgive you..in light of her son's actions...
..If my son had done this.... I would have responded with a tearful thank-you...because I would have such a load of sorrow & grieving, not only for my own son, but for the sons and daughters of ALL the other families!!!!!!