Because guns are evil and scary and racist and stuff!
At one point, he accidentally locked himself out of the car, with the gun inside. He may have thought this would lead to his capture.
Aiko was too scared to pick up the gun. She eventually unlocked the door and let him in. He did unspeakable things to her and murdered her.
Had she been comfortable with guns, she may well still be alive today.
And the five people he subsequently murdered would never been victims.