The mother did "nothing"?? What do you expect her to do? The kid is already 20 years old. Up until he went off the deep end by shooting up a school, he never committed a crime. It's obvious he had issues and some possible warning signs in the past, but we can't lock up people for thinking something.
Should we just implement some sort of "Minority Report precog psychics" and lock up people for crimes they have not (yet) committed?