Why did Kaplan betray Reddington?

According to Kaplan, she betrayed Reddington because he betrayed Elizabeth Keen and begs him to let Liz Keen go.

Kaplan betrays Red by helping Elizabeth Keen fake her death. Red initially shoots Kaplan, however, she survives due to a metal plate in her head. Her loyalty to Reddington was rooted in her love for Katarina and a desire to protect Elizabeth.

Mr. Kaplan, whose actual name is Kathryn Nemec had worked for Raymond Reddington for over 20 years. She helps Red with cleaning up crime scenes and evidence disposal.

“The Blacklist” is an American crime thriller television series that originally premiered on NBC on September 23, 2013.

The show follows Raymond “Red” Reddington (played by James Spader), a former U.S. Navy officer turned high-profile criminal who voluntarily surrenders to the FBI after eluding capture for decades.

The series was created by Jon Bokenkamp. On February 22, 2022, the series was renewed for a tenth season, which is scheduled to premiere on February 26, 2023.