Ex-Streamwood officer James Mandarino testified that he feared for his life during the traffic stop that was caught on a dashboard camera last year.
The officer's alleged victim, Ron Bell, says he didn't do anything to deserve a beating.
Mandarino is also accused of using a stun gun on a man who was a passenger in the vehicle.
The 11 seconds of videotape taken from Mandarino's squad car led to his firing and criminal charges against him. The 42-year-old defended his actions in court, telling a judge that he struck Bell 15 times with a baton because he feared for his life.
In the pre-dawn hours of March 28 of 2010, Mandarino said he observed a black SUV driving recklessly. Mandarino followed the vehicle into a driveway. The officer later learned it was Bell's home. Mandarino said he drew his weapon when Bell and his passenger, Nolan Stalbaum, refused to get back into the car. Mandarino testified he was forced to tase Stalbaum twice after he disobeyed several times. Mandarino said Bell and Stalbaum were verbally abusive and appeared intoxicated. Mandarino testified he labeled the traffic stop high risk and called for backup.
At one point, Bell was on his knees outside the vehicle with his hands in the air. Mandarino said Bell disobeyed orders several times to get all the way to the ground. Mandarino testified, "it was a split-second decision where I felt it was necessary to strike his right arm with the baton."
During cross examination, Mandarino said for the very first time that he thought Bell was reaching toward his gun so he kept striking.
While Mandarino said he get on the police radio to call for backup, he admitted he never said he was being threatened.
Mandarino is charged with aggravated battery and police misconduct. If convicted he faces three to five years in prison.
This is a bench trial so it will be the judge that will decide the case.
The defense has not rested its case yet, although closing arguments may take place Tuesday afternoon.