Andreas Lubitz is not a unique case, despite all the rhetoric from all sides who should know better. Depression in individuals is common today in every level of society and for multitudinous reasons. It can go so deep that a person can feel that the only way out is suicide.
It has been stated that Lubitz had a mental breakdown when undergoing pilot training in America. With skilled help, people do recover and go on to success, but alarm bells should have rung at Lufthansa to give him a different job.
The mental canker must have been so deep-rooted that it was buried from sight.
A grudge had developed, for whatever reason, that he had to show the world what he was capable of.
The people in the plane were not considered as such.
Although this behaviour is abnormal it is not potentially unusual.