They stole the ending from an old Roger Corman movie, Forbidden World.

And it didn't make much sense either. In that movie, they expose the alien to cancer tissue so it'll absorb it, and it instantly dies, instead of just getting sick.

In this one, they expose the alien to blood with sickle cell anemia (how'd this guy get selected to be an astronaut with an incurable disorder like that?), and it instantly dies. It made even less sense here, though, because the alien already had its host. It just avoided taking over people with health issues like sickle cell anemia, which makes some sense, actually, but it doesn't go around absorbing DNA from everyone it comes across or anything. Nothing is established in the movie universe to explain why having a few sickle cells enter it's bloodstream immediately cause it to totally melt down.