A small percentage of the human population are sociopaths and psychopaths. I’ve heard that 1 out of every 25 people is a sociopath.
The Trek mirror universe humans, which are typically referred to as Terrans, are crueler than their normal-universe counterparts. In fact it’s confirmed in ENT’s “In a Mirror Darkly” that Mirror universe humans are historically more cruel and violent. The TNG novel “Dark Mirror” gives a historical comparison between normal and mirror universe humanity too.
I’m not psychologist, but I think the condition of sociopathy and psychopathy is much more prevalent with mirror universe Terrans than it is than normal humans. If one out of 25 humans in the normal universe is a sociopath, my guess is that maybe one out of 10 Terrans in the Mirror Universe is a sociopath. Even though the behavior of mirror universe Terrans might have a lot to do to their cultural environment and societal upbringing, I suspect the minds of Mirror Universe Terrans are somewhat different and are like that of sociopaths and psychopaths and don't feel empathy like normal humans, which I guess even includes mirror universe Terrans who have human counterparts in the normal universe.