I think the reason for the cynicism in certain characters like Fifield is to make Shaw stand out more. She's optimistic about the mission, and that creates an interesting dynamic between the characters.
* The issue here is; what the viewer knows is not = to what the characters know/believe.
- The viewer knows there will be space alien monsters in the movie/story.
- The two scientists, Fifield and Milburn, do not expect that and they do not believe in that possibility.
* This is based on Vickers who set up the mission and who hired the two scientists.
- Vickers expected to find nothing important on LV-223 as she stated in the movie.
She was convinced no giant space aliens would be found.
She hired a geologist and biologist who were completely against Shaw's theory.
- The two scientists made this clear in the staff briefing.
* Logically the two scientists not only didn't believe in giant space aliens, they had no interest in giant space aliens.
- This is why Milburn walked away from the decapitated Engineer.
- This is why Fifield says to Shaw that he had no interest with giant space aliens.
They were hired because they agreed with Vickers and not Shaw.
* But Milburn was interested in primitive life forms like worms and snakes.
This is why Milburn tried to examine or even capture the Hammerpede.
Wildlife experts do that kind of thing in our world, for research or for putting animals in zoos; And several of them have been killed by poisonous snakes in non fiction.
It is plausible that this can happen to a character like Milburn.