It's kind of amazing to watch this, with or without previous knowledge of Lovecraft's story upon which it was based. It has an engaging start, but also an irritating one, and it spends too much time of its similarly too-long running time poking about needlessly. Character motivations switch instantly and arbitrarily, sometimes from one scene to the very next. The script is all over the place, and because of this it doesn't manage to give the viewer anyone to really sympathise with or care about.
Witley, Karloff's character, is hostile and needlessly secretive, and in fact attracts more questions than he would if he were more charming and less desperate. Stephen, the supposed 'hero' of the piece, is uselessly assertive -- he only insists when it's either too late or it means nothing. And the 'heroine', Susan, should change her surname to 'Witless', because that's about all there is to say about her character. She's utterly worthless and stupid, even practically throwing herself into mild danger to be rescued. Unfortunately, Stephen is almost as bad, forgetting his intent to leave in the very next scene after resolving to, now for some reason determined that no questions remain unanswered...and then of course, there's the scene where he nearly hurls an axe into Susan's head! All things considered, she probably would have been okay. Seriously, this woman had a plant sneak up on her.
There are some very good parts, though: the mysterious Helga, the horrifying monstrosities in the greenhouse, and the strange scorch mark left in Merwyn's room. The final menace, while a bit silly, was at least striking. But it's too uneven and too scattered to be suspenseful or exciting, and it just manages to feel more perfunctory than anything else. It would have been far better to end it simply; the way it ended likewise brought up so many more questions than it needed to, and there was both little stake in the futures of any of the surviving characters and little explanation as to how they intended to go from there.
It's suitably off-putting that the villagers are so reticent and hostile, but at the same time it also doesn't really make sense. True, ignorance and prejudice are rarely logical, but being so aggressive to someone who doesn't know and not even being willing to speak of it? What did they believe would occur if they just told him up-front what had happened, as far as they knew it? And for that matter, how long had it been since that happened in the first place? There's really not a sense of time in the film, much less a scale of time having passed. The doctor says he hasn't been working for some time, which is confirmed by his assistant; how would Susan not have known about any of this?
Some last points: never, ever use that epitaph for any tombstone; a man who cannot stand on his own is unlikely to be able to dig and fill a grave without assistance; after Merwyn collapses, meals are never again mentioned and none of the characters seem to eat for the rest of the film; the greenhouse is not attached to the main house and the door was left open; OF COURSE IT CAME FROM SPACE. And those are just a few of the most obvious points to make.
This is the kind of film that you could watch to pass time on a rainy Sunday afternoon, but don't watch it expecting to be riveted or excited, or least of all frightened. Karloff's charm and natural charisma are really all that manage to keep this watchable, and it is a testament to his ability. Unfortunately the rest of the film is just not up to par, with a disorganised script, unlikable characters, uneven performances, and no real suspense at all. It's a pity, because one can see the elements of a good film in there...they just didn't come together here. Also, The House at the End of the World is a far superior title to the insane Die, Monster, Die! A more accurate title along those lines would be Die, Mother, Die!.
Review by moonmonday from the Internet Movie Database.