There was lots of human experimentation by the Americans during World War II, but most of it was much more consensual, less horrific. Volunteer conscientious objectors were used for research on a starvation study at the University of Minnesota, for example, as part of research funded by the Office for Scientific Research and Development.

The most problematic work though was in the area of nuclear technology. It was new, there were key safety questions to be asked, the whole thing was being massively expedited, and yet it was all kept secret to an unprecedented level. This meant that informed consent was not seen as an option.

So at several sites, coordinated by of the Manhattan Engineer District (the Manhattan Project) there were human radiation experiments that involved injecting terminally-ill patients with substances like plutonium, to see how it was excreted by their systems. It also involved passive monitoring of people working with plutonium, uranium, and polonium to see how their bodies were processing the toxic and radioactive metals. There were concerns, shared amongst people working on the project, that this work was in many ways unethical, because they did not have any intention of telling people if they had gotten too-large doses, especially in places where the workers in question did not have knowledge of what they were handling.

Separately, there is also the issue of testing a surface-burst nuclear weapon, knowing it will produce downwind fallout, in an area that had low population density but not zero population density. The scientists and security agents of course had no intention of telling the surrounding populace that they were being silently irradiated at low levels. The scientists were acutely aware of the possible dangers, and stationed MPs and scientists in nearby towns with Geiger counters, hoping for the best.

There were other issues of this nature, this bad mixture of secrecy with safety. The small "water boiler" reactor at Los Alamos, for example, vented radioactive gases into a canyon nearby, but its vent carried no warning labeling on it or restrictions — something that made the safety officers very uncomfortable.

Eileen Welsom's The Plutonium Files talks about the plutonium injections, and the reports by the Advisory Committee on Human Radiation Experiments, from the early 1990s, contain discussion of other WWII-era unreported exposures. Most of the exposures were pretty low, but it is still pretty unethical. The people who took over the atomic work in the mid-1940s explicitly talked about this as having violated the Nuremberg conventions, and kept it secret explicitly in order to avoid causing scandal.

All that being said, in my view this is pretty piddling compared to Mengele and Unit 731 — unethical, but it lacks the horror. And there is a "great good" being sought-after here; knowing how plutonium was excreted helped set safety standards, for example, for the thousands of workers who had to deal with it. It wasn't arbitrary, in the way many of Mengele's sadistic experiments were, trying to sew together twins and the like. But it was still unethical, still a dirty deal made under the cover of war, and this is not just the judgment of hindsight, but how many people who knew about it thought about it at the time.