Jurors deliberated for eight hours over two days before rejecting Ronald Lee Haskell's insanity defense. His attorneys had argued that Haskell believed voices in his head were telling him to kill the Stay family at their suburban Houston home in 2014.
Jurors will next hear evidence in the trial's punishment phase before deciding whether to sentence Haskell, 39, to life in prison or death.
MORE: 5 years after murders, man accused of killing Spring family execution-style goes to trial
Prosecutors alleged Haskell was motivated by vengeance and had plotted to hurt anybody who helped his ex-wife, Melannie Lyon, after she left him. Lyon testified that Haskell physically abused her and their children, so she moved them all from Utah to Texas to be with her family after the divorce.
Authorities say Haskell traveled from California and stalked Lyon's family for two days before killing six of them.
Cassidy Stay, who was 15 at the time of the shooting, was shot in the head but survived by playing dead.
The now-20-year-old woman testified that she prayed and begged her uncle "please don't hurt us," but that Haskell forced the whole family to lie face down on the living room floor before shooting them one by one.
WATCH: Cassidy Stay takes the stand in her uncle's trial
Among those killed were 39-year-old Stephen Stay and his 34-year-old wife Katie, along with their children 4-year-old Zach; 7-year-old Rebecca; 9-year-old Emily; and 13-year-old Bryan. Katie Stay was the sister of Haskell's ex-wife. Haskell was convicted in the deaths of Stephen and Katie Stay.
After the shooting at the Stays' home, Haskell tried going to the houses of his ex-wife's parents and brother, but officers took him into custody after a long standoff.
In Texas, an insanity defense is rarely used and seldom successful.
"You may stop and say to yourself, 'Well, anyone that kills that many people must be insane,' but that's not necessarily true," said legal analyst Steve Shellist. "You can have someone that kills people that knows what they're doing is wrong, so they may have some other mental health issues. So they can still be found guilty, because they're not insane as the law defines 'insane.'"
A forensic psychiatrist testifying for the defense said Haskell wasn't responsible for his actions because of severe mental illness that prevented him from knowing right from wrong. The psychiatrist testified Haskell was suffering from a form of bipolar disorder, a brain condition that causes unusual shifts in mood, and from schizoaffective disorder, a condition characterized by hallucinations or delusions.
Prosecution experts testified that Haskell did not have a severe mental illness and had faked his symptoms. He knew his actions were wrong and had carefully planned the killings, they said.
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The Associated Press contributed to this report.