ODESSA, Ukraine – In the bloodiest day in Ukraine since government snipers claimed more than 50 lives during the EuroMaidan Revolution on Feb. 20, 46 people were killed in Odessa on May 2. The violence started when an armed group of pro-Russian activists attacked a peaceful pro-Ukrainian rally, whose participants, uncharacteristically, fought back. But nearly a week later, many questions remain about who died, how they died, why so many perished and who is responsible for their deaths amid accusations from Ukraine’s leading presidential candidate, Petro Poroshenko, that a toxic chemical was used in the building where most lost their...