ethics of life

In the course of our discussion section for one of my classes today, someone raised the issue of whether some of the stem cell research we were discussing were ethical or not and at what point researchers should stop such research, even if our capabilities exceed such a point.
This got me thinking about a number of issues. I remember reading about a scientist (unfortunately the name, nationality, affiliation, and even branch of science escape me) who was involved in the development of Agent Orange but who was later absolutely horrified to see its use during war. He spent the rest of his years working toward pacifist goals. This leads me to wonder to what degree scientists can actually control the use of their work and at what point society preempts scientific information (rightfully or not); that is to say, to what extent can or should a researcher foresee the possible uses of his/her work, and to what extent does the researcher has a responsibility to stop unintended or unethical uses of it?
The other issue that popped into my head was related to some of the debates about stem cell research in the public sphere. Not once has anyone even thought to question the ethics of science and technology directed at destruction (instead of creation) of life. No one protests the development of more and more advanced weaponry - the development of the atomic bomb, while controversial, was loudly lauded during and after its time. We might make a stink about Saddam Hussein's WMD's, but what makes it ok to continue researching and producing our weapons, many of which are too dangerous and secretive for the public to even know about?