As a boy of about 6 years of age seeing this at a drive in

I remember two scenes that scared me:

1. A man being shot to death in slow motion at the beginning
2. A man supposedly being buried alive (who i thought was the same man, LOL)

I remember those 2 scenes and those 2 only. So it turns out it was this movie
And I thought I never saw any R-rated movies until I was about 14. I think I fell asleep the rest of this, because it was the second feature and I don't remember watching it