Hawaii Island Police charged 18-year-old Forrest Keesler with second-degree murder on Sunday in the death of his father, Jeffrey Kelly.
KHON2 reports that according to court documents, the teen and his dad had gotten into an argument on Saturday. Keesler then shot Kelly with a pellet gun, and the father fell to the floor, hit his head and stopped breathing, according to the outlet.
Kelly was taken to the hospital, where he died shortly thereafter.
Keesler's stepmother, Angela Bracamonte, told Hawaii News Now that Keesler is severely autistic and sometimes violent.
Bracamonte told Hawaii News Now that that Keesler's mind was "fixated on one thing and one thing only": "He's always talked about how he wanted to kill his dad."
Bracamonte also said that Kelly was an alcoholic and that he would sometimes agitate the teen, who needed help.
"When he's not in his violent mode, which only his dad can get him in," she said, "he's a wonderful kid for real."
During his first court appearance on Tuesday, KHON2 reports that a judge ordered Keesler to undergo a mental evaluation.