Short Answer

Many Muslim scholars have discouraged Muslims from working in the defense industry because much of that technology is used to kill or harm innocent people through various destructive wars. However, the technology made by these companies also helps humanity by providing commercial airplanes, and by securing our country, etc. So it is not a black & white issue. If possible, it is best to stay only with a company that does not produce technology that will be used to harm people. The next best is to only work in a division of a defense company that is unlikely to be misused to harm innocent people.