Story highlights Thomas Yoxall says killing a man is hard to reconcile

He tells reporters he couldn't stand and watch as man attacked state trooper

(CNN) Thomas Yoxall says there was no time to think, no time to be scared. A state trooper was being attacked and he just couldn't stand by and do nothing.

"I had to help. I knew I had to help," he told reporters in Phoenix on Tuesday. "To me there was no choice."

It happened very quickly as Yoxall drove west along Interstate 10 just west of the small town of Tonopah. Yoxall saw the emergency lights of a state patrol SUV and flares on the ground. He spied a man on top of trooper Ed Andersson, a 27-year veteran, and got out with his gun.

Yoxall said he told the attacker to stop. The assailant didn't and Yoxall killed him.

"This is something that I will live with, but I wouldn't change it, because another man got to go home to his family and his family gets to keep him a little while longer and that's the important part," Yoxall said of the January 12 predawn incident.

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