Weapons researchers develop new ways of waging war. While in some cases new weapons will be more targeted and less harmful than the old ones, in general we expect this work to be very bad for the world. Over the medium term new weapons technologies becomes widely disseminated and so both sides end up being able to use them. Usually this makes any remaining wars more destructive.
Furthermore, weapons researchers are among the most likely to accidentally design new technologies that could be used to accidentally destroy humanity, such as nuclear or biological weapons in the past. We just don’t know what new opportunities for mass destruction are available, and we are probably better off not knowing.
https://​​80000hours.org/​​2015/​​08/​​what-are-the-10-most-harmful-jobs/​​