Is someone whose behavior shifts from caring and thoughtful to cruel likely to be suffering from a personality disorder?
I shared confidences with a male coworker I had grown close to as a friend but he suddenly stopped speaking to me. Even worse, he taunted me after we were assigned to work on a project together and pushed me so I tripped and made disturbing remarks that were so unlike the person I thought I knew.
Everyone has a personality, which is the enduring way one experiences oneself, other people and the world..
If one's personality consistently gets one in trouble, that usually is a sign of a personality disorder. In the case you describe, it would only be evidence of a personality disorder if 1) the person has been like that since adolescence or early adulthood, and 2) it is manifest in many situations and with different people.
Without knowing the person's overall pattern of behavior over the long term, one cannot say whether this is evidence of a personality disorder...although it might be.