Film about people trapped in a mansion/house (circa late 1960's-to-mid 1970's)

[PleaseHelp123]

Not a lot to go on here, but hopefully one of you movie wizards can tell me the name of this film.  I believe it was released circa the late 1960's to mid 1970's.  It centers around a group of people who find themselves trapped in a house/mansion.  They are being killed off one by one.  Also, there may have been a doberman pincer or some other guard dog that presented an additional problem as far as escape (though I'm not positive about this dog aspect).  In any case, my most vivid memory is of the very last minute of the film.  Two of the characters eventually escape the house.  They run up towards a car parked outside (I think it was on a dirt road near a wooded area), relieved that they can finally make their escape.  BUT, one of them pulls out a gun and shoots the other.  End of film.  Does anybody have a clue as to what movie that might be?  I know the details I've given are so generic, and of course, that's why it's been so difficult for me to track it down.  I hope one of you can tell me the title of the film or point me in a direction.  THANKS IN ADVANCE!!!