Given that I'd rather travel back in time and erase the invention of gunpowder..
I'd invent a bullet that injects a compound into the person, which kills them if their testosterone levels are too high (all freakishly violent type males for example) and decrease testosterone levels in people with high to medium levels (most scientists and calm males have low counts) and increase in people who have far too low counts (men with little to no testosterone are usually annoyingly wussy, women with higher testosterone are usually skinnier, and more sexually active)
Wars would stop, people would think more or have sex more.