Simple. Compare BB to HBP.
Do pitchers hit batters simply because they are wild or do they hit batters because they are mean? The Meanness Factor simply compares BB and HBP. It's pretty basic and indicates general intent.
A few years ago I applied the meanness factor to individuals. Among post WWII (after 1946) the meanest pitchers were Don Drysdale and Pedro Martinez with Jim Bunning third. There are two biases: sidearm pitchers who tend to miss in or out rather than up or down (see Sandy Koufax) and pitchers who walk so few batters that their meanness factor is skewed (see Mariano Rivera).