Very vaguely I recall the plot. It may well be a B-movie, or maybe I just didn't find any familiar faces then in the movie. There are only 3 characters in the movie. Following are the plot points I remember:
1) A couple drive down to their cabin/cottage, and on their way they give ride to a hitchhiker, and later allow him to stay with them at the cottage/cabin.
2) The wife is, evidently, not happy with the marriage, and soon starts to seduce subtly the hitchhiker.
3) The husband seems to suspect the affair.
4) Right up till the end, we are made to believe that the wife will somehow influence the hitchhiker to kill her husband so they both can be together.
5) However, the twist in the tale is when the hitchhiker kills the wife instead, and it is revealed that he is the husband's lover, and the whole thing was just a setup to bump off the wife.
The plot may seem a tad identical to Sidney Lumet's Deathtrap, but that's not the movie.