Relatives of Buffalo’s supermarket-slaughter suspect are copping the COVID defense, telling The Post on Monday that the teen likely snapped because of his paranoia and isolation from the pandemic. They added they had no clue their kin, accused 18-year-old Payton Gendron, was an alleged white supremacist and said he clearly needed help after threatening his high-school classmates almost a year ago — although they aren’t sure he ever got it.