She either loved him from the start or fell in love with him later on, she probably just wants to stop him from going on his mission to keep him safe now.
What is up with these comments? You come to a manga about a boy starting a relationship with his teacher and now everyone develops a guilty conscious when it's a girl and her teacher????
That would be a nice twist, I'd much prefer that over the obvious "dad tries to stop mums revenge without knowing that those kids killed his son" trope.