The genocide that swept through the central African nation of Rwanda in 1994 is among the greatest horrors of the 20th century. In roughly 100 days, Hutu extremists slaughtered some 800,000 people. In the decades since the massacres, Rwandans have enjoyed some peace and stability. Many in the West credit Rwandan President Paul Kagame, the former rebel leader. But Michela Wrong, author of a new book called “Do Not Disturb,” says the truth about Kagame is far more sinister.