He resigned from the hospital in 2015 in lieu of termination.  He was unable to find work afterwards and was living in a homeless shelter as a mental health occupant before going on his shooting rampage. 
Dr. Bello had three previous arrests on his record at the time of his death.  He was arrested on August 28, 2004, in Manhattan following an incident in which a 23-year-old woman said an assailant grabbed her crotch, held her arms, lifted her up and carried her off, saying, “You’re coming with me." 
His victim, Dr. Tracy Tam, was covering a shift for a coworker and was not Bello's intended target.
Bello had threatened to kill several people at the time of his termination in 2015 at the hospital. His intended target was a resident who was not at work the day of his shooting spree.