It is contrary to justice to allow someone to murder simply because they forgot to take their medication. That is called getting away with murder. Consider this report from the New York Times:
Lawyers for a schizophrenic man accused of pushing a woman to her death in front of a subway train opened their case yesterday by unveiling voluminous medical records to show that the defendant had such a long history of unprovoked attacks against strangers … Mr. Goldstein, 30, has pleaded not guilty by reason of insanity in his second-degree murder trial in State Supreme Court in Manhattan. (“Subway Killer’s Defense Cites Past Attacks”, New York Times, October 19, 1999.)
A schizophrenic who randomly pushes a stranger in front of a train is no less guilty of murder than a testosterone-charged punk who fires a hail of bullets into a rival gang member’s house. Both took actions they knew would be lethal, and both can claim chemical imbalances in their brains that drove them to do it.
As medical science progresses, more human behavior will be ascribed to neurological predestination. Defendants will argue that the structure of their brain, as determined by their genes and environment, made them do it. But if these arguments are taken to their logical conclusion, no one could be held responsible for any crime. The only clear line that can be drawn is to insist that people be responsible for their own conscious actions.