Why is it immoral to allow people to take a risk that they explicitly consent to (ensuring they have full knowledge of the potential consequences), but not immo

Why is it immoral to allow people to take a risk that they explicitly consent to (ensuring they have full knowledge of the potential consequences), but not immoral to let hundreds of thousands or millions more people die that you could have potentially saved if you undertook the challenge trials? A few bad cases and deaths that were consented to, versus hundreds of thousands to millions of cases and deaths from people that didn't consent. This morality scale doesn't make sense.