@JibbaJab
Yes, its the total unpredictability of NPD that I find difficult. Both types can be harmful, toxic, nasty, abusive, etc etc, but like I said before, I can almost comprehend the thought processes of psychopaths because they tend toward clinical, no-nonsense, black or white decisions, whereas NPD is often totally irrational and will create merry hell over perceived slights that simply do not exist, and it's sometimes only well after the fact and with a lot of distance that you realise just how disturbed and removed from reality NPD 'logic' can be.
A previous OP mentioned psychopaths often having a strong moral code, and very firm sense of justice, or what is 'right and wrong', and I would agree with that. There is always an arbitrary element to it, and even then I can understand that because I personally struggle to empathise with any situation that I have no personal experience of, but I will fight tooth and nail for anyone that has been through a circumstance that I have personally experienced and felt was unfair or unjust. It's not a 'sense of justice' in the same way that people think of legal justice, more a 'personal concept of natural justice'.
With NPD, there is no 'set of rules', because the grievance will be an outrage to them one day, but then they will happily administer the same treatment to another person the next day, with seemingly no recognition of the fact that they were distraught about experiencing the same thing themselves. What is claimed to be right or wrong is completely scattergun and changes on a whim.
With NPD I've always felt that there's an arbitrary pettiness, contradictory willingness to play hard and fast with what is right and wrong, and calculated degree of malicious, devious, nastiness that isn't necessarily there in psychopaths, but I would follow that up by saying the spite doesn't always manifest itself as big, grand, long-term schemes to undo a 'rival', it's often just ridiculously petty, infantile, immature, and see-through nonsense, but NPD's seem to live for it, which I just can not understand.
As well as being dispassionate and detached, I also have absolutely no tolerance for 'drama', but the NPD people I spent time around seem to thrive on it and positively require it in order to have a purpose, whereas I just laugh at the petty, infantile, simple-minded nature of it and walk away. Drama-llamas I just roll my eyes at, because I honestly can not see the point myself, yet both NPD's in my life would be described as "Drama llamas" by anyone who knew them reasonably well.