Disney always did do a number on stories. The first of her thrillers I read was This Rough Magic. It probably contributed to my lifelong fascination with Greece (also looking at you, Gerald Durrell and Mary Renault). The one I reread most often was Madam Will You Talk, though. They all read pretty dated now, but then so does Jane Eyre.
As to the Merlin stories going dark, the source material was at least as dark. I can live without a conventional HEA if the writing is good enough, which it is imo.