Self Defense/What is a sociopath?
You wrote that Tim Roth's character in 'Rob Roy' scared you... I fail to see why... I never saw the complete movie but from the few clips I found in Youtube, he came off more as a selfish 'douchebag' than as a sociopathic monster... What is your definition of a 'sociopathic monster'? If selfish douchebag = sociopath, then a lot of people (myself included) could qualify as sociopaths at a time or another... 'Normal people' don't kill other people, don't rob other people and don't rape other people... but, beyond that, behavior-wise, there is not a great deal of difference between Archibald Cunningham and 'normal people'... Besides, wasn't he completely caught offguard when Liam Neeson's character grabbed his rapier in the final duel at the end of the movie? Ultimately, he isn't that scary... ;)
In Dune, the original novel Frank Herbert wrote in 1965 (not the crappy sequels his son wrote), after the Harkonnens and the Sardaukars defeated the Atreides on Arrakis, Paul Atreides and his mother Jessica were forced to flee in the desert to find solace among the Fremen... They met a band of Fremen led by Stilgar... Among these Fremen was a Jamis who urged Stilgar to kill these 'non-Fremen' and 'take their water'... Stilgar was about to do so when Jessica used her 'Bene Gesserit skills' to overpower him while Paul fled, knocking Jamis in the process... Jamis was pissed off (I am a great Fremen warrior and a mere kid beat me!) and wanted to engage Paul in a knife-duel to 'avenge his honor' while Stilgar was capable of swallowing back his pride and to ask Jessica how she beat him so he can become a better Fremen warrior...
If I understand your previous explanations well, does that mean that Archibald Cunningham will be more akin to Stilgar than to Jamis? While the 'typical sociopaths' you met through your life were more akin to Jamis? And this is the reason why Archibald Cunningham stood out from a 'typical sociopath' and scared you?
Well since we're using movie references. Archie is to a douchebag what Godzilla is to a lizard. With the latter example both are reptiles, but one is much bigger, destructive and more dangerous. Same goes with the former
But more than that we can use a logical construct. All sociopaths are douchebags, but not all douchbags are sociopaths.
Sociopath/anti social personality disorder:
Sociopaths/anti social personality disorder are (functionally) incapable of empathy. It's not that they choose to ignore it with some people and have it for others (like douchbags do). They don't have it. It -- along with a lot of other normal traits - is missing from their make up.
If you watch the whole movie you'll see an overwhelming pattern of anti-social personality behavior with the character, everyone is a target for his machinations. The nature of his behavior changes according to their social status, but he uses everyone.
There is no negotiating and compromise with these fuckers, they can only be deterred. This by the immediate danger of consequences (e.g. you WILL shoot him in the face if he crosses you). The idea that it's wrong and hurtful others doesn't enter their heads. There is no 'future' or 'maybe' threat that works(e.g. if you do that you'll go to jail). If they think they can get away with it, they'll do it.
Fortunately -- as I said elsewhere -- they tend to be lazy about acquiring skills especially about violence. Instead relying more on
1) their willingness to act
2) speed of action, and
3) other people's hesitation to use violence
For example, in Rob Roy Archie robs and murders Rob's friend. Ordinarily, I'd say it's Hollywood-ized, because Archie confronts, chases and then kills the friend (who he and someone else set up to be carrying cash instead of cheque). A normal sociopath would just wait in the bushes and shoot him in the back. Or approach him smiling and shoot him.
Archie on the other hand had a rare attribute among anti-social types, the self-discipline to develop skills. Mad skills. Like the ability to use a smaller faster sword against a heavier one. (Your angles and deflections have to be perfect) It was his game. Also he liked to play with the people he was going to kill. He could do so because of his mad-skills.
That personality disorder with mad skills is what is scary. There's no way to deal with them but to kill them or drive them from the area. (Which is how he ended up in Scotland in the first place).
Also because of their skills, you're going to bleed taking them down. In fact, there's a good chance you're going with them when you take them out. You gotta be ready for that. While going up against anybody is dangerous -- and you have to accept that can you can go down -- you train to increase your chances to survive. This is the guy that you go in knowing that you ain't gonna be walking out again. But you still gotta go.
Archie was surprised because he finally met someone who was willing to do it.
The Janis/Stilgar issue is different. That's more tribalism. An attitude of anyone NOT of my tribe is not human. They will kill outsiders.
Although Janis was only around for a few pages, in his memorial the Freeman were talking about what he had done for them and how he'd helped etc., etc. So he was integral to the tribe and group. That is something a sociopath is incapable of doing.
He also had a stable, committed relationship with a woman back at the sietch. Again not common with anti-social types. Cunningham's banging the maid and abandoning her when she got pregnant is typical.