I came across this page lately: Psychopathy - Wikipedia, the free encyclopedia and when I read all the characteristics of psychopathy I cound't stop thinkng that they describe Lisa. Just take a look, I added my comments:

3 Characteristics

3.1 Lack of empathy - checked (with Sam in Hell and mouring Dean it was the best year of her life)
3.2 Shallow emotions - checked (e.g. she felt little or no fear of the danger - monsters etc.)
3.3 Lying and manipulation - checked (oh, yeah, she manipulted Dean by using her kid)
3.4 Impulsivity, irresponsibility - checked (she irresponsibly invited stranger to live with her and her child)
3.5 Low tolerance for boredom - checked (having sex with random guys)
3.6 Poor behavior controls - checked (see point above and insulting others e.g. Sam, Dean)
3.7 Remorselessness - checked (lack of remorse, she blamed others (Dean) for her failures e.g. for the end of their realtionship- )
3.8 Egocentricity and grandiosity - checked (she put her needs above her son's)
3.9 Intelligence (Hare and Neumann (2008) state that there is at most only a weak association between psychopathy and IQ.) - checked, in Lisa's case it was a weak association
3.10 Perceptual/emotional recognition deficits - checked (she coulnd't recognize Dean's feelings while looking at him or listening to him)

The girl from the last episode, Melanie... She was so.much.better than Lisa

IKR? You almost wanted Dean and her to have a thing Nothing like that happened when we had to endure Lisa.