First, one should consider that this is not necessarily morally wrong by all standards of morality. One of the great moral dilemmas humans face is the degree to which the good of the majority outweighs the good of any one individual. One typical way philosophers analyze this is through what is called the "trolley problem," in which pushing one person in front of a runaway trolley would save the lives of several. Assuming, for a moment, that the scientists are correct in their estimate that one death will save thousands of lives, there is nothing inherently wrong in making that trade-off. If one took a Utilitarian stance, in which morality is based on "the greatest good for the greatest number," their decision would be morally positive. The main moral issue would be fairness in selection of those people to die.
A Kantian type of duty ethics would argue that one needs to perform moral acts guided by a sense of individual moral duty alone. In that case, one could say that the scientists as individuals have an absolute duty to preserve rather than destroy life, irrespective of the consequences. Assuming that the scientists were Kantians, one could appeal to their sense of moral duty, arguing that while they could not be responsible for large-scale outcomes of their actions, they would be responsible for making a choice that violated their desire to preserve life.