How about, lack of empathy?
I don’t think so.
As an isolated factor, I don’t think lack of empathy best nails the sociopath.
Many millions of people, after all, lack empathy and aren’t sociopaths. Also, exactly what constitutes empathy is a subject of some disagreement. Some LoveFraud members, in fact, question whether sociopaths even lack empathy (some asserting, to the contrary, that the sociopaths they’ve known have used their capacity for empathy to exploit them).
But the biggest problem with lack of empathy is its weakness in explaining the single, truly best signifier of sociopathy—the characterological exploitiveness of the sociopath.
It is a high level of exploitiveness that most singularly exposes the sociopath.