The sci-fi/horror franchise Species introduced the world to Sil, a man-made fusion of alien and human DNA with an insatiable need to breed. There have been lots and lots of movies about alien invasions, and it's no surprise why. Daily life tends to be rather mundane, and there are few fantastical scenarios as appealing and/or terrifying as making contact with a being from beyond the boundaries of Earth. Some fictional aliens are friendly, others are hostile, and some wanna get to know human beings in an altogether more intimate way.
Aliens in movies and TV shows sometimes come to Earth for the primary reason of having sex (and possibly reproducing with) humans. Some aliens accomplish this task by force, while others prefer the seduction route. However, just because an alien seduces a human, doesn't mean that alien has benevolent intentions for after the act is complete. Species villain Sil is most definitely part of the latter group, as anyone who hooks up with her is basically guaranteed to be killed once the deed is done.
Interestingly enough though, Sil actually isn't entirely evil. Her backstory is one of unethical scientific experimentation, fear and misunderstanding, and lack of proper preparation for what she eventually became.
Species: Sil the Alien's Origin & Powers Explained
Sil's existence is the product of communication that took place as part of the SETI program designed to make contact with intelligent alien life. Unfortunately, aliens did talk to a team of government scientists, first sending them instructions on how to make an endless amount of fuel. Naturally, the scientists then assume the race they're speaking with must be friendly, so when they're then sent instructions on how to reproduce alien DNA and combine it with a human, they foolishly do so. The result is Sil, who at first seems like a normal girl, but ages rapidly, is prone to violence, and possesses superhuman speed and strength, as well as a formidable healing factor.
Realizing too late what they've created, the Species scientists attempt to kill Sil, but fail, and she escapes, wreaking havoc and seeking suitable mates. Once done with them, or otherwise threatened, Sil kills whoever poses her danger, able to shape-shift between her human appearance and true alien form at will and use various stabbing appendages. She's not necessarily malevolent, as she operates purely on biological instinct, and while able to quickly amass knowledge, has been given no real reason to value human life. Once Sil accomplishes her mission of becoming pregnant, she gives birth very quickly, through the chest cavity. She's ultimately killed by being set ablaze, but it's unclear if anything else could've taken her out.