BOSTON — The guilt phase of Boston Marathon bomber Dzhokhar Tsarnaev’s trial was considered a slam dunk for prosecutors, especially after his lawyers bluntly admitted during opening statements that he participated in the deadly 2013 attack. But the outcome of the next phase of the trial is much more difficult to predict. The same jury must decide whether Tsarnaev, 21, should be put to death or spend the rest of his life in prison. The penalty phase begins Tuesday in U.S. District Court. Debate over …