There is plenty of vapid speculation going on about the possible motives of Andreas Lubitz. Lots of people are depressed, and people sometimes kill themselves, but it's obviously uncommon to take a lot of other people along for the suicide ride. Not, however, unprecedented. These researchers found 24 cases of "aircraft assisted suicide" in the U.S. from 1993 to 2012. Most of them were private planes and probably were solo ventures; however the story recounts 4 cases of commercial airline pilots deliberately crashing planes and killing passengers. In no, case, however, did they leave behind an explanation. (One pilot survived, and he was clearly psychotic.)
A somewhat different, but possibly related case, is that of Harold Shipman, a British general practitioner who is estimated to have murdered 236 of his patients, and likely more, by means of fatal injections of diamorphine, which is the name the British give to prescription heroin, which is a thing there. He was convicted of 15 murders and sentenced to multiple consecutive life sentences. He eventually killed himself in prison. Even though they had him dead to rights, he never confessed and never offered any explanation for his actions.
Unlike the murderous airline pilots, self-destruction was evidently not originally on Shipman's agenda. However, though we can only speculate, it is possible that the murder half of the motive was related. Shipman is no the only medical professional who was into murdering patients. Michael Swango is an American internist who murdered as many as 60 patients and colleagues. He murdered his colleagues by poisoning their food, but he murdered his patients much as Shipman did, with drug overdoses. According to an essay by Joseph Geringer (at the link),
Forensic Psychologist Dr. Jeffrey Smalldon (who has examined many serial killers, among them John Wayne Gacy) believes that Swango's drive is "a preoccupation with control and manipulation," including a feeling of control over life and death itself. Sans a formal examination, Smalldon says Swango displays the common threads of a narcissist. . . .Lubitz, of course, didn't get to savor the pleasure of killing after the fact, but he did take several minutes to enjoy it -- he could have just pointed the plane straight down. I suspect that he wanted to go out with a feeling of power and control. Alas, we will never know.
The most chilling evidence was Swango's own diary in which he said he killed for pleasure. He loved the "sweet, husky, close smell of indoor homicide." He claimed that these murders were "the only way I have of reminding myself that I'm still alive." How does one explain a man going through the arduous path of getting a medical degree and then using it to launch a career of murder and assault on the people who entrusted their lives to him? One can almost understand the attributed motivation of serial killers of murdering to have complete power over an individual. But killing for pleasure?