Most important mental illness?

What is the mental illness that would have affected the most lives in history (and the "ongoing history")?
I think it could be one that could be described as the one that makes people believe that it's acceptable to use force (or unprovoked violence).
I know that ideas are evolving in regards to addiction. It's related to an individual's exposure to violence (here the usual term used is "abuse") in the first years of life. I hope that one day the "aggression" illness (the one I'm alluding to here) will be understood in the same way.
In my opinion, this concept needs to be explored as it's the most important mental illness of all time, including today.
It's understanding can provide the most important basis for improvement in all aspect of our lives and future generations.