There don’t seem to be any, except that some acts of evil are inscrutable and beyond our understanding.
Many of the assumptions people had about Adam Lanza and his murders are undermined by the official report, according to Jacob Sullum at Reason. For example, Lanza had played a lot of video games but only some were violent. He had Asperger’s, but had never threatened or even hinted at a desire to harm people, and Asperger’s is not linked to an increased incidence of violence anyway. His mother had the guns legally, but banning them would not have prevented his acts or even slowed him down, because his rate of fire was not particularly fast.
Lanza was from a family of divorce, but many such killers are not (for example, Columbine’s dark duo were from intact families) and divorce is common although mass murderers are not. Lanza did consider Columbine an inspiration, but that’s more a symptom of his troubles than a cause.
Were antidepressants a factor? The report indicates there were no drugs in Lanza’s system, and he was not on any drugs regularly, antidepressants or otherwise:
Reportedly the shooter did not drink alcohol, take drugs, prescription or otherwise, and hated the thought of doing any of those things.
Although I haven’t read the report itself, except for a tiny bit (it’s very long), I came across two paragraphs that contain quite a bit of information:
The mother did the shooter’s laundry on a daily basis as the shooter often changed clothing during the day. She was not allowed in the shooter’s room, however, even to clean. No one was allowed in his room.
The shooter disliked birthdays, Christmas and holidays. He would not allow his mother to put up a Christmas tree. The mother explained it by saying that shooter had no emotions or feelings.
Note the phrase “would not allow.” Lanza was twenty years old and lived in his mother’s home, but it seems he dictated quite a bit and his mother acquiesced in order to keep the peace. She may even have done this on the advice of a health professional.
But it’s not at all clear that any other approach Lanza’s mother could have adopted towards him would have helped or changed a thing. I offer as evidence that last sentence in the above quote: “shooter had no emotions or feelings.” Emotions and feelings cannot be taught if a person lacks them, nor are their absence the result of anything a parent has done or not done. If Lanza was a sociopath/psycopath—and it appears he probably was—and secretly inclined to violence but giving no indication of it, it is unlikely that anything could have been done to prevent his crime.