Hinkley was obsessed with the movie Taxi Driver, in which the main character -- a drifter like himself -- saves a teenage prostitute from her pimp through violence. Before his assassination attempt, Hinkley wrote a letter to Foster detailing how he was going to attempt to kill the president to win her attention (he had already sent her poems and love letters). Hinkley refused to cooperate with his defense attorneys unless they asked Foster to testify, enabling him to be in the same room as himself. "All the government psychiatrists concluded that Hinckley was legally sane -- that he appreciated the wrongfulness of his act -- at the time of the shooting. All three defense psychiatrists diagnosed Hinckley as psychotic -- and legally insane -- at the time of the shooting. Further evidence of the severity of Hinckley's mental problems came in May, two days before his twenty-sixth birthday, when he attempted suicide by overdosing on Valium. In November, he tried again -- this time hanging himself in his cell window" (Linder 2008). Hinkley's suicidal gestures, his absorption in the world of the film Taxi Driver, his fixation...

Finally, a "CAT-scan of Hinckley showing widened sulci in his brain was 'powerful' evidence of his schizophrenia: about one-third of schizophrenics have widened sulci, but only about 2% of the normal population" (Linder 2008). In response, the prosecution stressed Hinkley's ability to engage in foresight and planning regarding the assassination and suggested that although he may have been mentally ill, he still understood his actions were wrong. Yet even according to the strict, federal definition of insanity, Hinkley's words in a letter to the actress before the shooting suggest that he genuinely felt he was saving Jodie Foster, despite the fact that she barely knew him other than the odd letters he sent her.
Reference

Linder, D. (2008). The trial of John Hinkley Jr. The University of Kentucky. Retrieved:

http://law2.umkc.edu/faculty/projects/ftrials/hinckley/hinckleyaccount.html

Definitions of insanity provided in original case study.