Jonathan Joss, the “King of the Hill” voice actor shot and killed Sunday, was a deeply complicated man whose demons helped lead to his tragic fate, his ex-girlfriend tells TMZ.

Brandy Trejo, who dated Joss for 13 years during the height of his career, tells us … Joss was the most “self-destructive person” she ever met … and to her, it seemed like he couldn’t stand it when something good happened to him.

What’s more … Brandy says she witnessed him drive his neighbors crazy for years with his outbursts and outlandish behavior … so when she learned a neighbor allegedly killed him in an argument — she was not shocked.

As we previously reported … Sigfredo Ceja Alvarez bailed out of jail on Wednesday, and will be confined to house arrest awaiting trial, after he allegedly fatally shot Joss Sunday evening — he’s been charged with murder. The actor’s husband, Tristan Kern de Gonzales, called the shooting a homophobic hate crime … despite Tristan’s statement, San Antonio police declared there’s no evidence of a hate crime.

As Brandy explains … Joss was a loose cannon who liked to make trouble — obviously not an excuse for killing him! — and she believes he had undiagnosed mental health issues that he only made worse with drugs and booze.

She wasn’t just his girlfriend from 2006 to 2019 … Brandy tells TMZ she became the actor’s unofficial assistant and handler during that period of his career … and it was up to her to cool him off when he went into a random rage on a set.

Joss was best known for playing John Redcorn on the animated TV sitcom “King of the Hill” … but he had nearly 50 TV and film credits to his name, including in huge film productions like “True Grit” and “Magnificent Seven.” And Brandy tells us … they were kicked off a number of those sets — ’cause Joss would start trouble with a star or a director.

Brandy’s memories with Joss were not overly positive — she says he was very emotionally abusive and destroyed her self-worth — and yet, there were pockets of happiness when he would be a good man and take her on his fun Hollywood adventures.