Was it really that there were no warning signs of her mental and emotional illness or did the parents just miss them?
There is always some warning signs but if the parents are busy with work or have too many of there own problems it might be hard for them to spot. There is too much violence shown in TVs and on the internet these days.
Of course, parents should have noticed something strange in the behavior of his daughter. But complete insanity, might not be. Perhaps the girl, had a nervous breakdown at the last moment. In any case, the psychology is very complex and the common man, the part might not be noticed.