CARDIFF, Wales — He was born in Singapore, but his family moved back to Britain when he was a child. He had family problems and was known by locals as belligerent and aggressive, with a drinking problem. He had Muslim neighbors, who described his behavior as fairly unremarkable, and his children had Muslim friends. No one on the cul-de-sac in Cardiff, Wales, where Darren Osborne, 47, lived could readily explain what he is believed to have done: rented a van, driven it 150 miles to London and plowed into a crowd of Muslims as they finished prayers at the Finsbury...