** SHOW CHANGE **

DESCENT INTO MADNESS: BARBARA WALTERS LOOKS AT THE JARED LOUGHNER CASE THROUGH GRIPPING MOMENTS FROM INTERVIEWS WITH ICONIC KILLERS AND THEIR PARENTS, ON TONIGHT'S "20/20"

Plus: "20/20" Investigation: Scandal Inside the Peace Corps � Are They Putting Women in Dangerous Situations and Putting the Corps' Reputation Ahead of the Safety of its Volunteers? Brian Ross Reports Exclusively

Also: Elizabeth Vargas Interviews Ron Reagan, Jr. on the Centennial Anniversary of the Birth of His Father, President Ronald Reagan;

The tragedy in Tucson last Saturday has many people asking about the hidden mysteries that would turn a mentally disturbed man into a murderer. Over the years Barbara Walters has interviewed the shell-shocked parents of killers, as well as the killers themselves, including Mark David Chapman, John Lennon's killer, who heard voices; the parents of John Hinckley, the would-be Reagan assassin; and Sam Manzie's parents, who tried unsuccessfully to commit their disturbed teenage son before he committed murder. What signs were missed? Could alleged Tucson killer Jared Loughner have been stopped? Barbara Walters' report, which includes iconic moments from these past interviews, airs on "20/20" TONIGHT, JANUARY 14 (10:00-11:00 p.m., ET) on the ABC Television Network.

Plus: The Peace Corps is one of the most iconic and respected organizations in the world. Started by President John F. Kennedy, its very name embodies the best ideals of service: to help people. But on the eve of its 50th anniversary, has the Corps put young women volunteers in dangerous situations -- and then told them to keep quiet about it, all to protect its reputation? ABC News Chief Investigative Correspondent Brian Ross has an exclusive investigation into this allegation and the murder of young Peace Corps volunteer.

Kate Puzey was a beloved Peace Corps volunteer in the African country of Benin when she was murdered. Her family and the people in the village where she served still ask, why would anyone want to kill Kate? The "20/20" investigation uncovers a scandal within the Peace Corps involving Kate's attempt to blow the whistle on a Peace Corps employee she suspected of abusing seventh-grade girls.

In the second part of the investigation, Ross reports that more than 1,000 female Peace Corps volunteers have been raped or sexually assaulted in the last decade. As with the Puzey family, some of the victims say the Peace Corps has sought to cover up the incidents and treated the victims with insensitivity, more concerned with the agency's image than the volunteers' health. Peace Corps officials tell "20/20" the agency has a commitment to the safety of its volunteers.

Video preview: http://abcnews.go.com/Blotter/video/peace-corp-volunteer-nightmare-12582789

Story preview: http://abcnews.go.com/Blotter/peace-corps-gang-rape-volunteer-jess-smochek-us/story?id=12599341

Also: On the centennial anniversary of President Ronald Reagan's birth, Elizabeth Vargas interviews son Ron Reagan, Jr. about his notoriously conflicted relationship with his father, whom he describes as "easy to love but hard to know." Ron speaks candidly about lifelong ideological disagreements with his father, his parents' legendary love affair, the persistent rumors about his own sexuality, watching his father decline into Alzheimer's and his eventual death, President Reagan's conservative legacy and the current Republicans who would like to appropriate it.

Preview: http://abcnews.go.com/Politics/ronalds-reagans-sides-son-writes-memoir-mark100th/story?id=12584957

"20/20" is anchored by Elizabeth Vargas and Chris Cuomo. David Sloan is executive producer.