People have a responsibility to diligently consider the consequences of their actions, and to act based on their expectations of those consequences. "Consequence" isn't hard to define here; it's "what things are likely to happen after X, versus what things are likely to happen after ~X". There's no "will it be my fault?" There is only "will it be more or less likely depending on what I do?"

After the fact, the matter of responsibility is a function of whether the person took reasonable measures to obtain information, and whether the person acted in a way that would maximize the utility of the expected outcome based on that information.

Note that responsibility need not be a conserved quantity. Had the prosecutor not been informed that Swartz was a suicide risk, he would bear exactly the same amount of responsibility for his own death. The prosecutor would have just born less.