Various articles in the Huffington Post, the National Monitor and the International Business Times have discussed whether Dzhokhar Tsarnaev will testify during his trial.
I do not believe he will testify in the guilt/innocence phase or the penalty phase of his trial because that would open him up to cross examination, which he dares not risk. This does not mean, however, that he cannot speak to the jury. He has the right to allocution, which means he has the right to speak before sentence is pronounced. All defendants have this right.
In the first death penalty case that I tried, I had my client read a letter that he wrote to his daughter expressing regret for what he had done and for how that had affected their relationship. He broke down several times but eventually managed to get through it. The jury spared his life and several jurors told me afterward that they voted to spare his life because of his genuine tearful regret.
I am certain Judy Clarke and David Bruck have considered using allocution as a means for Dzhokhar Tsarnaev to speak to the jury about sentencing without being cross examined. Whether the tactic will succeed depends on whether he genuinely expresses remorse and regret.
The keyword is ‘genuine.’ Anything less and he will almost certainly be sentenced to death.
Frederick Leatherman Law Blog 
I doubt he will say anything that will help. If he has a chance to speak freely, he will not apologize or express remorse but instead use it as a bully pulpit to attack U.S. foreign policy. If he testifies in the guilt phase, I want to be there. It is something spectacular when a defendant takes the stand and testilies.
While Federal Rules of Criminal Procedure, Rule 32, seems clearly to state a right of allocution for a capital defendant, one rather surprisingly thing I’ve found out is that in a number of the federal circuits, there have been decisions holding that this right is not to make an unsworn statement before the penalty phase jury that will decide life or death, but only to make a statement before the trial judge who is about to pass sentence based on the jury’s penalty verdict!
Of course, if First Circuit precedent or practice is in favor of a commonsense interpretation that the right of allocution in a capital case is before the jury which actually determines penalty and can grant mercy, or if Judge O’Toole has ruled or does rule in this way, then the holdings of other circuits wouldn’t be relevant to this trial.
But what’s happening in other circuits may still be of interest in terms of how rationally or otherwise the federal death penalty is being administered. A recent Sixth Circuit decision, United States v. Daryl Lawrence (October 22, 2013), at 15-18, cites decisions of the Fourth, Fifth, and Eighth Circuits in support of its conclusion that a capital defendant, of course, has a right to address the penalty phase jury — but under oath, and subject to cross-examination.
My very quick web research didn’t tell me if there have been any circuit decisions interpreting Rule 32 more reasonably, although the District of Hawai`i, for example, has had some excellent decisions in support of the defendant’s right to make an unsworn statement to the jury which will actually decide life or death, rather than the trial judge later passing sentence who would be powerless to alter a jury’s death verdict.
I think the Ninth Circuit allows an unsworn allocution. We were doing that in Washington State and I never got any resistance from the prosecution or the judge, but I controlled how we did it to prevent opening the door to cross examination.
It’s my understanding that he expressed wanting to be a martyr on Twitter. Wonder if that sentiment has changed?
He surrendered, so he fell short of martyrdom and now it might be too late. I think one has to die in battle for the cause to achieve martyrdom. Not sure if execution after being found guilty and sentenced to death qualifies.
He did not insist on pleading guilty and asking the jury to sentence him to death, so I don’t think he’s a volunteer.