Rae Sremmurd's Mom Says Son Arrested for Murder Suffers From Mental Illness

Rae Sremmurd's mother says her son who's been accused of murdering his father and her ex-husband suffers from a mental illness requiring medication ... which she claims he's been off of as of late.

Bernadette Walker tells us her 19-year-old son, Michael Sullivan, deals with Schizoaffective disorder ... which lends to schizophrenia, as well as symptoms including hallucinations, delusions and mood disorders, such as mania and depression.

She says Michael was living with her in Atlanta until this past November, when she says he stopped taking his meds, leading to outbursts of anger that became more frequent. Bernadette tells us Michael eventually went to live with his dad, Floyd, in Mississippi.

While she tells us she doesn't know what caused Michael to allegedly shoot his father, she suspects an argument ensued ... and that Michael might've been off his meds again.

We broke the story ... Tupelo PD says Floyd was shot to death Monday, and that one of his adult sons was a person of interest. That turned out to be Michael, whom cops tracked down and had held for a psychiatric evaluation. A day later, he was arrested for murder.

Bernadette -- who's been divorced from Floyd since 2008 -- tells us she thinks the mental health system in this country is broken, because at 18 ... she couldn't require Michael to take his medication anymore, which she says was fully necessary to keep him calm.