MAJOR SPOILERS!! THIS IS FOR PEOPLE WHO HAVE SEEN THE MOVIE!!
Commenters have touched on the major theme of "honor" in the film, and too many comparisons to "Braveheart." I'll point out a few things about this movie that I have not seen other comments touch on:
This movie has a decidedly different take on abortion. The first character to get pregnant is the villain's (Roth) girlfriend, and when he coldly suggests an abortion, she states it is too late for that. The shame of her situation ("I'm to have a bastard's bastard.") leads her to commit suicide in a much later scene. The second character to find herself pregnant is Mary, Rob's wife, after a rape by Roth's character (and at least one sex scene with her husband, Rob). Late in the movie, as Rob is leaving for a final confrontation with Roth, Mary asks what she should do about the pregnancy of questionable origins, with a tone hinting of abortion. Rob replies in a noble tone, "it's not the fault of the child," and then states what he thinks the name should be, girl or boy. I find this "pro-life" stance on the part of the hero to be very un-Hollywood. Rob walks from the darkness of the house to the bright outside to make this comment -- not coincidental symbolism.
Another related theme is Roth's character is a bastard, someone who evidently does not know who his father was, and has few kind words for his mother, though he wears a picture of her in a case hung from his neck. Is it coincidence that Roth (devoid of family stability) is the walking definition of psychopath, while Rob is the strong husband/father figure, and of course the hero. In the final sword fight between Rob and the villain (Roth), the former slices the latter deeply across the chest -- the left side of the chest, over the heart. His employer and pseudo-father figure (John Hurt character) holds the mother's picture in his hand and gazes at it, before snatching it from the neck of the dead Roth.
Also what I find interesting was the direction of the rape scene, which was not quite graphic but neither was it off-camera and implied. I found it surprising in it's somewhat matter of fact depiction, with Mary convincingly showing the characteristics of someone going through the ordeal, and subsequent post traumatic stress (as we call it now). My point being that the rape was neither sensationalized nor just implied, which I find an interesting middle road for Hollywood to take.
In the final fight scene, I have to correct an earlier commenter: The weapon Roth chose was a rapier (or perhaps a short sword), the weapon Rob chose was a Claymore. Someone was really doing their homework on this entire scene. Roth would have the upper hand in such a situation, but of course the Claymore is a distinctly Scottish weapon. What is even more striking to me (as a fencer and someone who has read a bit on the subject) is that this final sword fight is one of the most convincing of any film ever made: The actors seem actually trying to kill each other -- not the usual slashes to the opponents blade we see in most movie fights (including the movies opening fight). Even more true to history, Roth is seen several times using the rapier as a thrusting weapon, which is it's purpose by design! (Rapiers were edged, but primarily a thrusting weapon with the edges used mainly for parrying an opponents thrust.) Rob uses the Claymore in broad slashes, as it's design intent. The fight goes down as I would expect it to -- Roth effectively wins. Though Rob wins the day by grabbing Roth's weapon (more symbolism) and striking him dead with a powerful slashing cut.
Folks, it is RARE to see this level of historical accuracy in a movie sword fight.
I'll also note that for whatever reason, I remember 1995 (the year of release) distinctly as a time of distrust of the U.S. government. Hollywood was obviously tuned into that, with the release of both "Rob Roy" and "Braveheart," and I think the anti-government leanings are why both films get so much comparison.
I think the different perspective that this film gives is refreshing to avid movie fans, tired of the same old, not so hidden messages from Hollywood.

Based on Ray Russell's dark bestseller, this John (WATCHER IN THE WOODS) Hough-directed bust has little going for it.
Though it does not lack gory violence, it lack narrative sensibility and "characters".
The "Incubus" of the title is a demon endowed with a mammoth penis that shoots red sperm into vaginas during intercourse -- or, to be more precise, rape.
John Cassavetes, moonlighting from his successful directing career, is convincing as a doctor who questions the circumstances of the bizarre attacks on young women.
Horrific possibilities of the victims spawning demonic offspring are not considered -- and neither is the audience's tolerance for slow moving garbage.
The script's reluctance to explore the dramatic repercussions of a fertile premise exemplifies the major problems with this vapid Big-Schlong-On-The-Loose exercise.