Leave in anger.  Irked. ‘Half cocked’  suggests a loaded gun that is half ready to fire, so leaving a situation half-cocked implies one is upset and may return in an angry reaction, thus becoming fully cocked. (Compare to Snippy, Ruffle your feathers or Leave in a huff.) {see 50 Levels of Anger video}

“When I told my son he could not borrow my car to go to the city, he yelled at me and walked off half-cocked.”