What tactics does Edgar Allan Poe employ in "Hop-Frog" in order to make this tale especially dark and sinister?
The biggest literary element used by Poe in Hop-Frog in order to portray the dark and sinister nature of the story is suspense. Much as in The Cask of Amontillado, the readers know that something is going to happen, but they have no idea what, despite given a small hint.
Poe starts with a very light description of things, explaining the king's preference for jokes and therefore the presence of jesters within his court. Even when describing Hop-Frog, Poe keeps his tone fairly light. It is not until the scene with the king and his council that we know something is going to happen. When the king forces Hop-Frog to drink wine and throws wine in Tripetta's face, the readers know that something is going to happen, though they do not know what. The small hint provided is covering the king and his council in tar and flax, but the reader does not necessarily know why that is important (much like the shovel shown by Montressor in The Cask of Amontillado).
Finally, the action takes place at the very end, and because of all the waiting and hints, the readers see just how sinister Hop-Frog's plan is, how dark he is.