When science-fiction and horror overlap, there is running and screaming, the towns people grab pitchforks and torches and the very idea of what it means to be human is called into question. At least, that’s what happens in some of the best movies that dip into both genres.

The age-old question of where the ethical line is in science and whether it should be crossed never ceases to generate new terrors as technology continues to develop. Killer cyborgs and androids? Out-of-control genetic mutation? Radiation-spawned monsters? All of these have have fueled some of our favorite sci-fi horror films.

If you’re new to this subgenre, here are some great movies to start with:

Boris Karloff in Frankenstein (Credit: Universal Pictures) Frankenstein (1931)

The original mad scientist’s experiment gone wrong, with a career-making — and wordless — performance from lead Boris Karloff as the monster. With this and the horror hit “Dracula,...