Peter Lanza says that there isn't a waking moment that he doesn't think about what his son did and he is absolutely sure that if he had been given a chance, his son would have killed him too. He thinks that the reason Adam shot his mother, Nancy Lanza four times was once for her, once for him (his father,) once for his brother, and once for himself.
In hindsight, there wasn't anything that he could have done to prevent his son's behavior. He took him to several mental health professionals that all simply said he had Aspergers, and they didn't see any violent tendencies in him and they sent him on his way.
Peter Lanza said that growing up, Adam was just a weird little kid, but it became clear in middle school that something was amiss when his anxiety increased, eye contact lessened, he became more socially awkward, he started having sleep problems, and the inability to concentrate. Everyone just said it was the Asperger's but Peter doesn't think that Asperger's caused this massacre. He says;
"Asperger's makes people unusual, but it doesn't make people like this."
Peter also confessed that Nancy had told her sister or her best friend that she was afraid of Adam, but still slept with her bedroom door unlocked and kept the guns in the house, which she should have never done if she really feared for her life.
Peter has offered to speak to victim's families, and three of them have agreed to do so. He said that one family even said that they forgave Adam for what he did, even though he took their only son's life.