[Xanderfox]

I think the name of the American translation of "PaRaSiTe" is spelled Parasyte. I read up on the wiki-page, their is a Anime too, and the aliens are depicted as nether bad or good, but generaly lacking emoutions or a concept of morality. So do develop emoutionaly and morality(often with their own ideas on what is acceptable) while others don't. As a result some kill only bad people, while others make no distinction, the Hero befriends on only to have to Kill him because he goes on killing sprees when people find out his secret because he doesn't understand discretion (he kills them in the open, then kills the witnesses, and kills the witnesses of those killings, and you get it). Another, a female, becomes a scientist and tries to find out her species origins.