Strangely enough, some experts are worried about the inherent ethical and legal ramifications of weaponizing the human brain. That link is to a 2012 report on the use of neuroscience for policing and military purposes, and it expressed concerns about turning our soldiers and civil servants into "super soldiers," with their willing brains "plugged directly into weapons systems" and turning them into "amoral, combative automatons." That actually sounds, umm, optimistic?
And then there's cognitive enhancement via electrical brain stimulation. It might sound kind of hideous and One Flew Over the Cuckoo's Nest-y at first, but this area of science (along with its associated health applications) could definitely wind up being a huge advancement for our species. Yet there remains much trepidation regarding how nations might use neuroscience in malignant ways, because of course there is. Even within the realm of science fiction, this kind of thing turns out OK approximately 0.0% of the time.