I like MST3K, but this movie really does deserve more credit than they gave it. The premise is pretty daring for 1957, and like the earlier film I Married a Witch, this movie forces males in the audience (and probably a few of the women) to feel physical attraction to, and thereby sympathy for, a witch. The actress is really very extraordinary to see. Sympathy in the 1950s audience is also created for a prostitute, another "bad" person, not only by emphasizing her looks but by making her (or, rather, her French incarnation) the most noble character on the screen. The people who sell their souls to Satan, and celebrate a forbidden mass, also come across as basically sympathetic figures, because of their miserable fortunes in a difficult, repressive society. And The Undead is almost aggressively feminist, even, with the bad witch appearing as a much stronger figure than Satan, and the male doctor who manipulates the poor prostitute ultimately looking quite awful. Apart from all this anti-social artistry, the atmosphere is evoked well, and the evocation of lawlessness and life-beyond-respectability is very effective.
I admire The Undead and encourage people to see the film without robots in the way.