Sil is an alien-human hybrid created by the Search for Extraterrestrial Intelligence program.
Known as test subject S1L ("Sil"), she was formed from the fertilization of a human ovum with a alien DNA. Sil was raised and monitered in a controlled lab environment until the scientists studying her attempted to kill her. Escaping her confines she broke out into Los Angeles where she evolved and fell to her alien instinct to find a suitable male and mate; becoming an intelligent but hyper-sexual predator.
She is the main character of Species.
While Sil clearly has human-like emotions, including to some extent a desire to be loved.
When she kills she does so out of instinct in an effort to defend herself or preserve her masquerade which she has to maintain to survive.