"Basically I knew his name was associated with evil. I didn't know the extent to which he had gotten away with at all," Efron told host Jimmy Kimmel.
"My main concern once I heard this story was not to glamorize what Ted Bundy did," he said. "He is a horrible person and was a horrible person, rest in whatever. He's not a good dude."
Efron compared his portrayal of Bundy to Christian Bale's performance in American Psycho, but got the name of the film wrong.
"We've all seen Psycho. The movie that, like, is the dramatic jump and 'Look! Now I'm a serious actor ... Blood, guts, I'm crazy -- now I'm a good actor.' Sorry, Christian Bale, that's not what happened. You're really good," he said.
Efron apologized again after realizing he got the title wrong.
"Sorry, I'm thinking American Psycho. And I'm sorry Christian Bale, now for two things," he said. "Christian Bale, you're a phenomenal actor. I look up to you."
Extremely Wicked, Shockingly Evil and Vile recounts Bundy's crimes through the eyes of his girlfriend, Elizabeth Kloepfer (Lily Collins). Bundy was executed in 1989 after confessing to killing a number of girls and women in the 1970s.
Efron had shared a photo of himself as Bundy on Instagram in November. The movie premieres Friday on Netflix.