"I was surprised to hear it, even though he tried it before in the past," Houston, a friend of Markoff in their days as students at the State University of New York at Albany, told "Early Show" co-anchor Erica Hill Monday. "I just assumed that it was behind him. I had mixed emotions about the whole thing. At one point he was a friend."
Markoff, the Boston University medical student who was arrested in April, was who advertised erotic services on Craigslist and robbing another.
CBS News's John Bentley reported that the Markoff case has prompted calls for Craigslist to drop its adult services advertising, but the company has refused, claiming it monitors the site for any illegal activity. Bentley said adult ads constitute more than one-third of Craigslist's revenue, according to one Internet monitor.
Markoff was being held at the Nashua Street Jail in Boston. A statement released by Boston Police Commissioner Edward Davis and Suffolk County District Attorney Daniel F. Conley said Markoff "was alone in his cell and all evidence collected thus far indicates that he took his own life."
Houston recalled a night when the two went out drinking with some friends, and Markoff when they were going home.
"At the entrance to the dorm, he shoved me up against the wall and tried kissing me," Houston said. "I couldn't physically overpower him to get him off. It was really scary because I was fighting against him and saying, no, Phil, what are you doing? And thankfully one of our other friends came up and had to pull him off of me."
Houston, who was speaking from Philadelphia, said, "That's not the guy I knew. It was a totally different side. I just blamed it on the alcohol. I thought, well, this isn't normally him. Yes, he can be awkward but he wouldn't do something like this.
"When I knew him, I didn't really see any tendencies towards this."
Asked whether there was anything that could have led Markoff to the crimes he's suspected of, Houston said she was at a lost. "Who knows. I wish he had maybe left something behind saying what happened."
Markoff pleaded not guilty in the April 2009 fatal shooting of Julissa Brisman, of New York City, and the armed robbery of a Las Vegas woman, both at Boston hotels.
Markoff was arrested days after his image was shown on surveillance video at the hotels.
Shortly after his arrest in 2009, Markoff was put on suicide watch after shoelace marks were found on his neck, reports CBS News station WBZ in Boston.
CBS' "48 Hours Mystery" producer Paul LaRosa, who wrote a book about Markoff called "Seven Days of Rage," said Markoff had tried to kill himself twice.
John Salsberg, who was Markoff's lawyer, said he was shocked by his client's death. At the time of the earlier suicide attempt, Salsberg said Markoff was having difficulty transitioning "from being free to being incarcerated."
Markoff's trial was expected in March.