Richard Martinez is shattered.
His son, Christopher Ross Michaels-Martinez, 20, was the last of six people killed in Friday's rampage in Santa Barbara, California. The suspected gunman, Elliot Rodger, fatally stabbed his roommates, police say, before shooting at random victims near the University of California, Santa Barbara campus.
Michaels-Martinez was shot to death at a local deli.
Read: Father of Santa Barbara Victim Sobs and Rails Against Son's Death
He was Richard Martinez's only child.
"Our son was the most important thing in the world to all of us," he told ABC News. "It's hard to imagine how things are going to be like now that he's gone."
Read: Santa Barbara Shooter Was Angry Girls Were 'Repulsed by Me'
Martinez said he was proud of his son. Christopher was smart and active, with plans to study abroad and then go to law school. He had plans for Saturday -- a breakfast to meet his girlfriend's parents for the first time.
While Richard Martinez is devastated, he hopes something positive will emerge from the tragedy.
"Nobody needs to own three semi-automatic handguns," he said of Rodger. "It doesn't make sense."
Read: Elliot Rodger's Trail of Carnage
Gun violence is a complicated problem requiring input from lots of perspectives, including mental health experts, Martinez said.
"What we're trying to do is try to understand why Chris died and try to make it mean something because ... otherwise it just seems so hard," he said.
Rampage Victim's Father Shares His Grief, Pain and Hope