Secretary of State John Kerry said North Korean leader Kim Jong Un's decision to execute his uncle, a mentor, shows the rest of the world just how ruthless and insecure he is.

Kim executed his uncle by marriage, Jang Song Taek, for treason Thursday, in a highly scripted event that was reported by the country's state-run news service. The news service characterized Jang as “despicable human scum” who has a history of corruption, womanizing, gambling and taking drugs.

Kerry, speaking to ABC's "This Week," said the execution shows the international community “how ruthless and reckless [Kim] is.”

“It also tells us a lot about how insecure he is to a certain degree,” Kerry said. “It tells us a significant amount about the stability internally of the regime with the number of executions.”

U.S. intelligence has identified a “significant number of executions taking place over the last month,” Kerry said.

“Most importantly, it underscores the importance for all of us of finding a way forward with North Korea in order to denuclearize the peninsula,” he said. “It's an ominous sign of the instability and of the danger that does exist.”

Kerry also said the execution reminds him of the type of killings former Iraqi leader Saddam Hussein engaged in for decades.

“I mean, it really reminded me of video that we saw of Saddam Hussein doing the same thing, having people plucked out of an audience and people sitting there sweating and nobody daring to move or do anything,” he said.

ABC's Martha Raddatz, who was standing in as host for George Stephanopoulos on Sunday, also pressed Kerry about Robert Levinson, an FBI agent who was reportedly working with the CIA in Iran and disappeared seven years ago.

His family has confirmed to ABC News and other outlets that he had ties to the CIA and is accusing the U.S. government of abandoning him and the Obama administration of giving lip service about their efforts to push Tehran for his release during recent negotiations over the freezing of Iran's nuclear program.

Kerry said suggestions that the Obama administration isn't pushing hard for his release “incorrect and unhelpful.”