Callousness is a poor facade to thin skin. Personality disordered people have deep, festering wounds that have never been properly addressed. Hurting others is a coping mechanism that releases pressure, momentarily, but doesn't address the cause. That's what cutting attempts, but it's directed at others, rather than self. Psychologically, it's the difference between lancing a cyst and squeezing out the ick without getting the sac that contains it, so it's never really gone - but the ick is now on others.