10. Dead Space's Necromorphs Are A Disease
In the Dead Space series, supernatural 'markers' - ancient alien artefacts that lead to outbreaks of vicious necromorphs - are responsible for driving the human population mad through mind control while spreading their alien scourge across the galaxy.
However, one convincing fan theory explains that these markers serve a completely different purpose entirely: They're a distraction. It's theorised that the necromorphs were genetically engineered by other beings, spread like a virus that incubates in the nooks and crannies of an environment and infects and takes over a host's body. Consequently they should be treated in the same way as any other biohazard outbreak: by locating and eradicating the source of the infection.
The markers are there to make sure that doesn't happen however, and the hallucinations and seemingly paranormal properties linked to these artefacts entwine victims in a fantastical, supernatural conspiracy to keep them from logically dealing with the outbreak. In reality it's just the viral infection messing with the host's brain, but while the humans are preoccupied with getting to the bottom of the 'supernatural' markers, the virus can go unchecked to wipe out most of the population.
There's no indication of who or what constructed these things in the first place, but it's argued that they were placed as traps for sentient species to stumble upon, with the unknown creators being able to wipe out their competition without ever having to deal with them directly.