So imagine a story written in first person where a group of scientists in an isolated location (either like a space ship or a research base on a different planet) discovers a parasite. You see it has the ability to duplicate living things, but one day it escapes and you find human bones hidden somewhere so you know it replaced one of you.
This is followed by complete panic, not being able to contact anyone outside -- everyone against everyone because you know you can't trust anyone except yourself.
That's when you get injured - your blood is completely black.
You are the parasite, but you feel normal because you inherited all the memories of the person that died, you didn't even know this happened.
Would you still be the villan in this story? Would you be able to continue 'your' life?