Arguably, the first fully formed work of horror-science-fiction was Mary Shelley's Frankenstein which was the first story to give a monster a grounding in the world of science. In horror-science-fiction there is a definite sense of man being the creator of his own downfall, thereby becoming his own monster.
I put forth that science fiction is definable by its need to ask questions and use questionable practices to reach the answers. There is often an element of Chaos Theory involved as events perceptibly change the course of human existence through small changes to the status quo. Essentially, what we try to control we eventually destroy. Alternative realities that are not far off from our own make up the majority of science fiction premises.
Now, where does the horror come in? I believe it comes when there is a specific, violent threat to the status quo. When the changes that I mentioned above take on physical forms (usually, not always) and become the aggressor in the situation. Taking that definition we can put the following films into the horror-sci-fi category:
28 Days Later
Village of the Damned
What makes these films both sci-fi and horror in my mind is the threat becoming violent. One film I want to talk about in particular in this post is Danny Boyle's Sunshine (2007) which is most definitely a science fiction film and, I would argue, a horror film. A crew is sent to reignite the sun with a massive bomb on the Icarus II spaceship after Earth lost contact with Icarus I. And guess who stumbles upon Icarus I on their journey? And is there maaaaaaayyyyyyyyyyyybe something still alive on that ship?
**Beware Los Spoilers**
The super fun horror plot comes at the end. Pinbacker, a crew member on Icarus I has made his way on to the ship and has begun sabotaging the mission in hopes of being "one man alone with God." This move takes us from psychological thriller to a full out slasher film. I know a lot of critics had a problem with the last reel of the film but for me Sunshine is about human nature. Humanity has come together to pitch in the remaining resources to create a bomb that would restart the sun and of course humanity gets in the way.
In science fiction we can most clearly see the prejudices against horror of being a lowly form of entertainment. Just because there's a killer on board does not automatically make a film bad. It makes it a challenging film. You don't have to love it but the film shouldn't be written off because of the story. While Sunshine and most science fiction films are based on the conceit that the world should be made better through advancements, there is a reason we test on animals first.