When there are group conversations about favorite movies, it often breaks down to discussing everyone's favorite genre. More than anything else, it seems that people's tastes are defined by their favorite genres of books, movies, video games, music, etc. There are drama nerds, thriller cravers, and of course, the genre that breeds the most obsessive fans: horror. Those thirsting for blood, knives, screams, and a monster or two are horror junkies.
But many don't consider that horror isn't a solo genre. It can be intertwined into other genres, much like how a drama can also be a thriller, or an action movie can incorporate comedy. It all comes down to specific criteria that each movie needs to meet in order to be considered a specific genre. For horror, it is fairly simple. Horror is meant to instill fear into the audience by any means necessary. Monsters, slashers, gore, and anything that is sharp and covered in blood are just some examples of what can make a movie a horror flick. It just so happens that those aspects lend themselves easily to science fiction as well.
Sci-fi is a genre dealing with fictional science and how it impacts the world as it is known to the audience. Aliens, metamorphosized animals, and those that infest are common themes. Think of stories like Mary Shelley's Frankenstein or even H.G. Wells' War of the Worlds to understand the essentials of sci-fi stories. Both of these, depending on the filmmaker, can lean hard into the horror genre as well.
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Ultimately, horror and Sci-Fi are sibling genres. Both deal with curiosities, beings not fully known to humanity. To summarize it, they both delve into the unknown. What happens if man creates a being unnaturally? What happens when aliens come to earth? What-ifs and the fallout of those scenarios are the subject of many classic sci-fi horror tales. It's the easiest way to write sci-fi as it doesn't take much real-world knowledge to write it, and it works the best for horror because there's not much that's scarier than the unknown.
Possibly the easiest way of seeing how a sci-fi turns into a sci-fi horror (rather than sci-fi action or other subgenre) is by taking a look at the Alien franchise. The Alien franchise might be one of the most genre-changing franchises out there, and almost every fan has a specific movie in the franchise they prefer. That is because almost every movie — especially the first three — have clearly defined sub-genres. Yet, every single Alien movie is still a sci-fi movie.
Comparing Ridley Scott's Alien and James Cameron's Aliens is the easiest way to find where a sci-fi movie evolves into horror, and how it can quickly no longer be considered a horror. These two movies share so many aspects. They have the same title other than one being plural; they have the same protagonist and antagonist; and they even share some of the same story beats. The biggest difference between the two movies is the type of presence that the Xenomorph has. In Ridley Scott's Alien, the Xenomorph is just that, 'The' Xenomorph. Scott makes the alien a singular force of nature, like a slasher villain. He pits this monster against those who aren't equipped to defend themselves nearly as well as they should be, all the while in a confined space. It's a pure blueprint for a great slasher-style horror movie. It is not many versus many, it is many versus one. That one is an unstoppable force, and it's winning.
Meanwhile, in James Cameron's film, it is an army of aliens that the characters mow through with guns and explosives. In Aliens, the characters know a significant amount more than their previous movie counterparts did. It is once again the idea that the unknown is more terrifying than that which is understood. In Aliens, Ripley and the other characters have an idea of the threat they are facing and what they have to do to defeat it. They're going on a 'bug hunt' and they're teeth. Do they know everything about them? No, but it's enough to bring the level of suspense about the creature down, and that extends to the audience.
Going into the first viewing of Alien, viewers have no idea what the titular creature is or what it is capable of doing. But going into the sequel is another story. Now, the audience has actually seen the 'monster' and knows what it can do for the most part. There are not even remotely the same amount of questions and/or fears. Instead, it's focused on the intense quest to escape and destroy the creature, making it more of a sci-fi action thriller than a sci-fi horror — even though it's still plenty scary. It isn't just the level of fear that makes a movie a horror. It can also be the knowledge and expectations held by the audience watching.