But Darrell Brooks Jr's new insanity defense could be a tough sell in Waukesha County, which is still recovering from the horrors of that November day.
Here's a look at how insanity pleas work in the Wisconsin justice system and what Brooks' attorneys would have to prove to avoid prison.
Court documents allege Brooks beat the mother of his child just before the parade began in downtown Waukesha on Nov. 21 because she hadn't bailed him out of jail several days earlier. He had been arrested for running her over with his SUV.
He then drove the vehicle into the parade route, ignoring police orders to stop, according to a criminal complaint. He crashed into people head-on and ran them over as the lay on the ground, the complain said. He finally turned off the parade route, left his vehicle and tried to get someone to let him into his house. Police captured him there.
Prosecutors have charged him with more than 80 counts, including multiple counts of reckless endangerment and six counts of intentional homicide. Each of the homicide counts carries a mandatory life sentence.
No potential motive has emerged thus far, although his attorneys have said the officers who arrested him noticed he smelled of marijuana and his eyes were red and glassy. His public defender, Jeremy Perri, didn't return a message.
Brooks initially pleaded not guilty. His attorneys asked Judge Jennifer Dorow on Monday to move his trial out of Waukesha County due to negative publicity. When Dorow refused he changed his plea to not guilty by reason of mental disease or defect.
That's Wisconsin's equivalent of an insanity plea. Brooks is essentially saying he was suffering a psychotic episode and didn't realize what he was doing was illegal or was incapable of obeying the law.