If Tsarnaev is guilty why have a trial at all?

The Boston Marathon bombing trial opened with the defense admitting Dzhokhar Tsarnaev's guilt this week. So why didn't Tsarnaev plead guilty? 

Boston Marathon bomber Dzhokhar Tsarnaev's lawyer startled a packed courtroom during opening statements at his federal death penalty trial by bluntly admitting that he committed the 2013 attack with his brother, raising some thorny questions, mainly: Why have a trial at all?

Some legal experts and a bombing survivor weigh in on the strategies at work and the reasons why the trial is moving forward:

Q: Why didn't Tsarnaev's lawyers convince him to change his plea to guilty if the defense acknowledges he did it?

A: "A plea of guilty would result in them waiving all their appellate rights. There are some issues — the venue issue, the makeup of the jury — that they might want to bring to a higher court, so that's another reason they probably wouldn't plead guilty," said Boston College law professor Robert Bloom. "Further, they recognize that this is a two-phase trial and they want to do what they can during the first phase so as to start to make their arguments for the second phase ... the penalty phase."

Q: Why put the victims through this?

A: Rebekah Gregory, a woman who lost a leg in the bombing, was one of the first survivors to testify during Tsarnaev's trial. Hours after her testimony, she posted a letter to Tsarnaev on Facebook, saying facing him in court actually helped her.

"TODAY...I looked at you right in the face...and realized I wasn't afraid anymore. And today I realized that sitting across from you was somehow the crazy kind of step forward that I needed all along," she wrote.

Q: Why not just have a penalty phase for the jury to decide punishment instead of going through both the guilt and penalty phases of the trial?

A: "A competent death penalty defense lawyer — during the first phase — will present themes that are relevant to guilt — like diminished capacity, diminished responsibility — that are also entirely consistent with what's going to be said during the mitigation (penalty) phase," said Eric M. Freedman, a death penalty specialist and professor of constitutional law at Hofstra Law School.

"Death Penalty Defense 101 is to present a unified theme through the guilt phase and the penalty phase. ... Therefore, you are ill-advised to argue in the first phase, 'My client is innocent,' and in the second phase, 'My client is very sorry for what he did.' That's completely ordinary and being carried out in textbook fashion here. They are doing precisely what they are supposed to do."