Viktor Bout, the Russian arms dealer known as the “Merchant of Death,” walked out of a U.S. jail almost two years ago in a trade with Moscow for U.S. basketball star Brittney Griner. Now he is back in business, trying to broker the sale of small arms to Yemen’s Iran-backed Houthi militants. The 57-year-old, whose life reportedly inspired the 2005 Hollywood movie, “Lord of War,” starring Nicolas Cage, spent decades selling Soviet-made weapons in Africa, South America and...