What isn't canon is how it manages to take control. The body, in most circumstances, would resist ANYTHING that wasn't the victim's DNA. So what if the parasite, as it travelled through the body, copied the host DNA and RNA? Then, in order to gain control, it basically gives the victim's CNS a series of hormone baths, effectively rewriting brain chemistry.
The first hormone bath pacifies the host. The second wakes up the instincts stored in the brain stem in preparation for bath three. Hormone bath number three nullifies those instincts as well as any individuality in the brain, thus returning the brain to a malleable, 'default' state. Hormone baths four and five bring the host deeper and deeper under parasite control. Hormone bath six completes the process.
At the end of hormone bath six, parasite and host are now symbiotes. Killing off one will kill the other.
But what if, immediately after hormone bath two, the parasite's growth was stunted, and then it was killed? And therefore, the third hormonal change couldn't wipe the brain clean, thus leaving the instincts awake?
Essentially, this fic wonders what would happen if a literally primal part Leon's brain woke up and didn't go back to sleep? How do you deal with something inside you that registers two sensations, and has two overarching drives: kill and fuck? Especially when your super-ego takes one look at the thing, says, "ohmigod EW," and shoves it