Downward Spiral: Mark Salling 'Devastated' By 'Glee' Costar Cory Monteith's OD Death

Mark Salling was fighting many demons before his child porn arrest and eventual suicide — but the troubles seemed to have started with his close friend and Glee costar Cory Monteith's overdose death in 2013, an insider told RadarOnline.com exclusively.

According to a pal, the 35-year-old was left "devastated" following his the talented star's tragic passing.

"He was normal before Cory died. But his personality changed — just from happy to just completely aggressive and angry," the friend claimed.

"It was just very back and forth. One minute he was fine and sweet and normal and nice, the next he was angry."

As RadarOnline.com reported at the time, Monteith succumbed to a heroin and alcohol overdose in July 2013, leaving his costars and girlfriend Lea Michele to mourn the tragedy together.

Nearly five years later, Salling's committed suicide by hanging, the L.A. County Medical Examiner's Office confirmed to RadarOnline.com. His body was discovered hanging in a tree near the Los Angeles river in Sunland, California around 9 am on January 30.

In 2015, prosecutors raided the actor's home where they discovered over 50,000 "sadistic" and "masochistic" images of underage children. He was due in court for sentencing on child porn charges on March 7.

The insider described Salling as being very "isolated" and feeling "abandoned" following the charges brought against him.

"Regardless of what he may have done, it is a loss," the friend said.

If you or someone you know is in an emotional distress or suicidal, please call the Lifeline at 1-800-273-TALK (8255).