NEW YORK — The relentless series of mass killings across the globe poses a challenge for experts trying to analyze them without lapsing into faulty generalizations. Terms like contagion and copycat killing apply in some cases, not in others, they say, and in certain instances perpetrators' terrorist ideology intersects with psychological instability. Some of the attacks, such as the coordinated assault on multiple targets in Paris last November, were elaborately planned operations by Islamic State adherents.