The data doesn't support your assertion. The centuries-long trend is a decline in homicide rates. It appears that, on the whole, our "intelligent brains" are quite capable of choosing to be a non-homicidal member of society. The homicidal deviants you mention are an inconsequential percentage of the total population.
I found this chart to be especially interesting. The US homicide rate was roughly constant throughout the 20th century.* At first glance, this contradicts the overall trend, but it's more encouraging when you look at the demographic changes. In 1900, there were 75 million people, and 40% lived in urban areas. In 2000, there were 280 million people, and 80% lived in urban areas. Urban areas have much higher homicide rates than rural areas, so it's quite impressive that the per capita homicide rate didn't rise dramatically.
*The drop from 1940 to 1960 was a combination of the Great Depression and WW2. Young men commit most murders, and you won't have many young men if your families are too poor to have kids or your young men are off fighting a war. Predictably, the homicide rate shot back up when baby boomers started becoming young men.