I thought the screen adaptation was rather weak
You're not the only one - the critics savaged the film at the time of its release. Here's just a sample of Variety's review:
"'Burnt Offerings' is a trite, murkily plotted, and talky supernatural pic, which will probably move quickly to double bills [MB Note: meaning it will quickly drop out of the limelight as the primary film attraction and will slip to the secondary on a double bill back when it was still customary for two movies to play together in a theater. Those days are long gone. ::sigh::]. Lots of doors fly open and shut, actors behave strangely, and eventually three people die, but it's a long 116 minutes. Karen Back and Bette Davis supply some marquee interest for the United Artists release, but none comes off well in the heavy hands of producer-director Dan Curtis..."
And that's actually one of the film's "good" reviews.