A Guatemalan migrant has been arrested for allegedly lighting a sleeping subway rider on fire in Brooklyn on Sunday morning — then watching as his innocent victim burned to death in what the police commissioner called “one of the most depraved crimes one person could possibly commit.”

The savage killing — which happened at about 7:30 a.m. on an idling F train at the Coney Island-Stillwell Avenue station — shocked commuters, MTA workers and NYPD Commissioner Jessica Tisch, who said Sunday that the heinous crime “took the life of an innocent New Yorker.”