Peter Rodger, the father of the UCSB shooter Elliot Rodger, opened up to Barbara Walters about the nightmare of realizing his son was a murderer, saying, "We didn't see this coming at all." In the exclusive ABC News interview, which will air in a special edition of 20/20, Rodger spoke candidly about how he's struggled since his son killed six people and wounded 13 more in Isla Vista, CA, this past May. "I wake up and it's, slowly, the truth of what happened dawns on me. And you know, that is that my son was a mass murderer." Watch a clip from the interview above, and see the full episode this Friday.