Before his 'spree'

Josh Voorhees has a Slate article about Lanza:

As authorities continue to search for Adam Lanza's motive, it looks like they might not be able to rely on the twenty-year-old's hard drive for help. Police tell multiple media outlets that someone— presumably Lanza himself— did his best to destroy his hard drive at some point before the shootings that killed twenty children, six elementary school staff members, and his mother.
The Hartford Courant with the details:

Two law enforcement sources said the hard drive had been removed from Lanza's computer and broken in pieces. They said that forensic electronics experts at the FBI will examine the drive in an effort to determine with whom Lanza corresponded electronically and how he otherwise used the device. One of the sources said that Lanza used the computer to play a violent video game in which life-like characters engage in graphic battle scenes.

Sources told ABC News, meanwhile, that the hard drive appeared to have been damaged by a hammer or screw driver.
Forensic experts don't yet know whether they'll be able to recover the lost files or not, although from the sounds of it they are cautiously optimistic that they'll be able to salvage at least some of the digital information. Given Lanza's apparent lack of an online presence, authorities hope that the hard drive's files, if recovered, will provide crucial details into Lanza's life and mental state in the lead up to the tragic school shooting.
The fact that the hard drive was smashed led at least one unnamed law enforcement official to suggest that it was proof that the Lanza's killing spree was planned in advance. "If he destroyed the computer, that means there are things on there that would concern him," the source told the Daily Beast. "The kid knew what he was doing. This was a planned event. There is no question about it."

Rico says of course it was premeditated; you don't just wake up one morning and decide "Hey, I think I'll go kill my mother and shoot up an elementary school today"...