The term 'KILLER ___' can be filled in with the crossword solutions APP (3 letters), CAIN (4 letters), LULU (4 letters), and SERIAL (6 letters). App is short for application, which is a computer program designed to perform a specific task. CAIN is a biblical figure who killed his brother Abel in the book of Genesis. LULU is a character from the German fairy tale 'LULU the Killer'. SERIAL killers are individuals who commit multiple murders over a period of time.