Because of the fact that State of Decay was written for a very different era indeed, it is the last of a breed of stories that hadn't been seen for some time already, and wouldn't be seen for a good while after this - Gothic horror. Any story that would threaten to frighten children after this would ultimately resort to a typical mid-80s gorefest.
And that's the interesting thing about the horror in State of Decay. Despite some truly gruesome makeup for the Three Who Rule (especially in their death scene, as seen above), and an entire vat full of blood, the real chills in this story are the ones that are implied and by the reactions of others to them. Scenes such as The Doctor and Romana's exploration of the vampire's larder, The Doctor's reading out of vampire legends, and the Three Who Rule describing the havoc they'll cause when they get out of E-Space are chilling, and yet all of the terror in those scenes are described without a single drop of blood being spilled.
The fact that the best scenes in this story are the ones where we have to use our imagination is probably a good thing, given the fact that easily the weakest sequence in this story is the depiction of the Hydrax turning in mid-air and returning to the planet to destroy the Great Vampire, which ends up looking like a paper cutout of a rocket and a glove puppet. Still, State of Decay is a real highlight of Season 18, and it's good to see Terrance Dicks winning out in his battle with Christopher H. Bidmead as the pedantic science is kept to a minimum here.