The neighbor, Lauren Vickers, tells TMZ she called emergency services for help after she saw 14-year-old Colt wearing the same clothes daily, and his 2 younger siblings being locked out of the house at night, screaming and crying to get in.

Lauren alleges she witnessed Colt’s parents getting into heated arguments outside the house multiple times, often in the driveway.

She explains that Colt's mother, Marcee, eventually moved out with the youngest, Coley ... while his father, Colin, stayed behind with Colt and Colt's younger sister, Jenny.

Lauren tells us the landlord who owns the family home was so fed up by their drama that he insisted if Marcee ever came back, the whole family would be kicked out -- and that’s what happened when she returned sometime later. Lauren claims she knows this because their homes share the same landlord.

Lauren says it wasn’t just home life where Colt struggled. Her daughter, who went to school with Colt -- not at Apalachee High School, where this week's shooting happened -- says she saw him being mercilessly bullied.

Colt’s aunt, Annie Polhamus Brown, responded to Lauren’s claims ... telling TMZ she can "absolutely confirm" his home environment was "toxic at many stages in his life."

Colt is suspected of fatally shooting 4 people -- 14-year-old students Mason Schermerhorn, Christian Angulo, and teachers Richard Aspinwall and Christina Irimie -- at his Winder, GA school. Nine others were injured.

Two days after the shooting, Colt appeared in court on Friday morning ... he faces the maximum penalty for 4 felony murder charges -- a life sentence without the possibility of parole.