What do you mean by held accountable? Do you mean legally? If so, that wouldn't make any sense because society obviously sanctions those deaths or they wouldn't happen. The President, congress, military (or particular military henchmen), and public (or prime minister, parliament, military, and public, or whatever one's country has) aren't held accountable for those deaths, and they're all complicit to some degree of directness or another.

If you mean socially accountable in some way, the above kinda still applies, except that it is actually possible for some hearts to hold accountable all of those people/institutions. So... should they? I think scientists are responsible to a degree, but it's not like they're fully responsible for the sum total of all the deaths as if they'd killed them with their own hands. Also, some scientists believe they're doing the right thing when they create weapons (see http://www.doug-long.com/einstein.htm , for example), and even if they're ultimately wrong, intentions matter a lot, or should matter.

If the scientists are just doing it for money, then, well, there's probably no need to hold them accountable because fate will do its work, and also because they're probably not aware enough human beings to fully realize what they're doing.

Though this raises a question, (again), what does it mean to hold accountable? Does it mean to judge and condemn? If it's not a legal measure (which I've already argued against), then it can't be punishment, so it must be judgment and condemnation, n'est pas? And I don't believe in condemnation. Everybody deserves support, in whatever form that may take--even if it means a good talking to.