Gregory Alfred, 25, told police he set out to find white people to slice back on Mar. 10 because he blamed them and “the system” for preventing him from freely smoking weed, sources said. The slasher chose to take out his anger on 53-year-old Janina Popko, and cut her across the neck as she was walking near the corner of Beverly and Rugby roads that morning in Ditmas Park, sources said. “He’s just a sick person,” Popko told The Post on Tuesday. “I’m very lucky to be okay. I am still shaky.” “I never was racial. I grew up in...