The criteria for doctors to force someone to be confined to a psychiatric hospital against his or her will is determined by state law.  In almost all states that means the individual is deemed to be an imminent threat to self or others on the basis of a mental illness.  Imminent is held to mean, and is reflected in the standard of care,  that the mentally ill person has very recently made such an attempt or communicated their plan to soon carry out that intent.  Yes, this very sick individual committed  a heinous act consequent to a mental illness, but there is no report that he communicated his intent to anyone and the hospitalization in question occurred weeks earlier I do believe.  The treating doctor and hospital are required, by law, to release that person once the imminent threat is deemed to no longer exist.  The key word is "imminent.".  The law determines the criteria, not the doctor.