I totally agree, Pluto does something to Venus that's extra poisonous, I think this is where the lack of empathy comes from.
I had close dealings with one a few years back. Not fun. Great learning experience.
To an experienced astrologer, what you need to do is paint a picture of someone's general conscience and empathy level on a continuum. That would give you a pretty accurate (astrologically) perspective of how that person is in a relationship. Even the professionals (psychology wise) use a spectrum and set of behaviors before they diagnose someone as a psychopath.
You need to view a chart as a painting, and each section a stroke of the paintbrush that adds up to the whole picture.
Based on my experience, stellium combinations in both Scorpio and/or Capricorn makes people ruthless, selfish, lacking empathy, and too willing to let personal ambitions override moral or other people's interest. But that section by itself is only a single paint stroke out of an entire painting (the chart), and while that may be relatively poor character, but it does not a psychopath make.
A weak or poorly aspected Jupiter is also low or no conscience.
Moon and Venus are relationship planets, and those in the signs of Scorpio, Capricorn, Virgo also lend to low empathy levels. Poorly aspected also means a certain coldness in dealing with people.
Pluto afflictions within angular houses lends to strong and unavoidable power and control issues, which may doom a relationship.
I've also heard angular or prominent Nessus means an abuser. Nessus conjunct MC, ASC, etc., not necessarily DSC. And sometimes Nessus in the center of a grand or T-square.
Those are just some signs of low empathy and conscience.
I dated a men for a year, who was moody, controlling, strong sex drive with inability to express it , no ability to connect emotionally, borderline sadistic, cheater, lair, manipulative, extremely egoistic and materialistic, stalker, selfish, womanizer, reckless, childish ....
Taurus sun square Leo moon, Venus opposition Uranus, Mars square Uranus, Mars conjunct Saturn in cancer, Saturn square Uranus, Jupiter opposition Saturn, to name few....
Mercury in Gemini opposition Neptune for the sweet talk and compulsive lying
Venus conjunct Jupiter for the extreme charm and wealth
My Saturn was conjunct his south node and Venus, his north node was conjunct my moon, my vertex was conjunct his moon. WE were deeply connected, but I could not take the abuse anymore.
I had a karmic feeling with him, and was not able to let him go. He was also not able to let me go for almost a year after the break up. He still pops up in my dreams occasionally and I feel that we are still connected.
Keep your eyes open and trust your intuition