It's not like it's unstudied new science, experiments on prisoners are such a well known bad thing because people did cruel experiments which treated the subjects as objects - this is choosing what's known to be an effective way to painlessly die over a more painful and less effective method.

And yes we know what's going to happen, nitrogen isn't a new thing and people have asphyxiated due to it before (over a hundred people in the US in the last thirty years), just not when it's been purposefully administered in a prison.