Maybe my phrasing implied too much causality, but a mixture of psychoactive drugs, extended sleep deprivation, and combat stress could turn anyone into a murderer.
And recent research absolutely has shown that heavy cannabis usage in adolescence and early adulthood does have a clear association with an increased risk of developing psychosis and schizophrenia. Just because the war on drugs lied about the effects of less harmful drugs like cannabis doesn’t mean they can’t be hazardous to your health like any other substance.
Regarding heroin - one may not be particularly murderous while zonked out, but addiction can make people surprisingly aggressive.
I’d also like to add that the company-grade officers stuck in the mud alongside enlisted were less at fault for the needless deaths than the incompetent general staff or the political actors who got us and kept us over there in the first place. You might enj the novel “Matterhorn” by Karl Marlantes, a Marine infantry officer who served with distinction in Vietnam. Or you might prefer “Dereliction of Duty” by Lt General H.R. McMaster, a renowned commander of the GWOT era who wrote his PhD thesis on the failures of Westmoreland and the other generals who mangled Vietnam.