[dekhn]

Control groups exist because they get us closer to global optimality.

We can't eliminate suffering and dying of untreated people but it's generally considered ethical to eliminate suffering and dying of many while withholding that treatment from a subgroup of the study ("allowing them to die" by not taking action, rather than the "causing them to die" by taking action).

as long as that selection is done randomly and "fairly" I think it's an entirely acceptable risk. There have been trials that were truly badly run, and people died and suffered more than necessary, due to mistakes (often amateur ones) in the protocol. I'm more concerned about those types of deaths.