Wrestling superstar Hulk Hogan was shocked when the WWE's Chris Benoit, 40, murdered his wife, Nancy, and son, Daniel, 7, then committed suicide the weekend of June 23.

"He was peaceful and kept to himself," he tells Us.

Hogan, who last saw Benoit in March, says he wonders what pushed the wrestler over the edge.

"I think it had to be something personal, a domestic problem between him and his wife."

And Hogan tells US the wrestler's wife (and WWE costar) had a dark side.

"She was into devil-worship stuff. It was part of her [wrestling] character, but [she was] somebody who gets so close to their character, someone who gets into their character too much. Sometimes these people believe their own publicity."