Ash (aka Hyperdyne Systems 120-A/2 android): Ash was the science officer on board the Nostromo in Ridley Scott's 1979 Alien. Being a true corporate man...er, android Ash deliberately allows an infected crew member aboard the Nostromo as per his secret instructions which states, "Bring back alien life form. Crew expendable." Boy, were they. However, the crew was quick to make Ash expendable first.
(Image courtesy of Wikipedia)