Not sure what you mean by killed it but didn't eat it? Did the snake attempt to swallow? If so, keep in mind that snakes have the ability to disengage the jaw to open wide enough to take large prey. Many times this requires re-positioning the prey to go down better and snakes will sometimes spit out their prey and then strike again. The open and closing of the mouth is often a sign of re-adjusting or re-attaching the jaw following swallowing. Also, my snakes often do this to clear the mouth of any debris they do not want to swallow, such as pieces of substrate. Along with this they may rub the side of the face along items in the cage to scrape debris from the mouth. In either case, the behavior should only be temporary.