Casteel told jurors Monday in Livingston County court that he was filled with fear and anxiety while in traffic, most likely from undiagnosed delusional and paranoid thinking. But the jury agreed with prosecutors who argued that his actions in the October 2012 shootings were deliberate and premeditated.
Casteel's sister sobbed loudly in the courtroom as the verdict was read.
"I'm just happy that everybody can be safe now. ... Imagine being shot at. It's not fun," one of the victims, Jennifer Kupiec, later told reporters.
The jury was asked to consider an attempted murder charge in the shooting at Kupiec's car, but Casteel instead was convicted of the lesser charge of assault with a dangerous weapon.
He didn't contest five gun charges. Police said they matched Casteel's gun to bullet fragments recovered from victims' vehicles.
On the witness stand Monday, Casteel admitted that he fired at cars along the Interstate 96 corridor, but that he "absolutely" did not intend to hurt or terrorize any of the drivers.
"I saw a long line of traffic, felt fear and anxiety and shot," Casteel testified, describing one of the shootings, which took place in four counties, forced schoolchildren inside during recess and had the area on edge for weeks.
"I can't testify to the number, but I did fire at cars, yes," said Casteel, who added that he kept a handgun on the floorboard near his right leg.
In one instance, Casteel testified that Kupiec, who had been tailgating him on I-96 then passed his car on the right, agitated him to the point that he picked up the gun, rolled down the passenger side window and fired at her car.
Under cross-examination by Assistant Attorney General Gregory Townsend, Casteel said he never thought about the ramifications of the shootings, only that he wanted "to send a message to back off."
He was the only witness called by defense lawyers.
Last year, he pleaded no contest but mentally ill to assault and firearms charges in Oakland County in connection to related shootings that took place there. He faces up to 12 years in prison Tuesday in that case. A no contest plea isn't an admission of guilt but is treated as such for sentencing purposes. The mental illness allows him to get treatment in prison.