I have tried Jack3d a couple of times and I did not experience any negative side effects. The only thing I can think of is the dosage.
That doesn't prove anything. Just because one person has used it without harm doesn't prove it's safe. Nor does the fact that a person who died used it sometime prior to his or her death prove that it's dangerous--I have taken a number of drugs, and I will die sometime, but that doesn't prove that any of those drugs are dangerous. I assume that the Harvard internist who said it is the most dangerous ingredient has sound reasons for saying so that are not detailed in this quote. The key would be proof that a significantly higher number of people who took it died, compared to those who don't take it. I assume that it's not the first time that Army trainees have experienced sudden death.