With due respect, the jury should not consider any effects of the sentence, aside from its appropriateness for the crime being considered. Their sole job, at this point, is to ensure that the punishment fits the crime, that is, is just.
This task is, of course, difficult enough, but they will be given arguments from each side and instructions from the judge. They will not be given a crash course in international terror, the likely response of lone nuts, or anything similar. No one will testify about whether the death sentence is likely to result in more terrorist actions against the US. You are suggesting that they make a decision based on factors that they are ill-equipped to know.
Moreover, if we base our sentencing on the likely threat of violence from third parties, where should it end? If a motorcycle gang threatens violence unless its members receive lenient sentences, should the jury be informed and take that into account? Should the threat of violence alter our views of the appropriateness of a sentence?
It strikes me that the jury's task is difficult enough, and that the aim of both the jury and the system itself ought to be choosing an appropriate sentence as far as justice is concerned. If we instead take into consideration any potential threats as a result of the sentence chosen, we are merely ensuring that such threats are effective and encouraging their use hereafter.
