I remember watching this on TV in early 90s. But the movie might be way older than that.
I think the movie takes place on an island. The protagonist is stranded there. Some sort of alien creature is killing people. Eventually the protagonist discovers a laboratory like room somewhere in a cave on the island. Then he founds an old tape and begins watching it.
The tape starts with some old black & white footage of a big spaceship crashed on the island. Two aliens are captured and are being experimented on by humans. The aliens look humanoid.
One of the aliens seem to be female. The other one is kept locked in a room with a big glass window and watches the humans from there. Then the footage shows scientists cutting the female alien and removing a fetus from her womb. Seeing this, the captive alien gets extremely angry and hits the glass window with his forehead, shattering the glass. The protagonist then understands why the alien was hostile. And the rest is not very clear in my memory.