Inconsistent, unrealistic

Rating: 5/10 (20 July 2017)

Is it difficult to write believable, rational scenarios and still inject drama? It's an honest question, not to say I know better.
But I find lots of movies or shows where the only way the writer progresses the plot or create an exciting scene is when the key characters do stupid or inexplicable things out of the blue.
This is not a review, just expressing my disappointment when I was so looking forward to this movie (having forgotten the forgettable Prometheus).
Halfway through this movie, the thought suddenly hit me "man, this is so c**p". The inconsistent characters and their behaviors were plainly irritating. People overacting and going apesh** when they're not supposed to, and showing little emotion when they should, things happen conveniently just to advance the plot, etc.
Human behavior is instinctive. Even children know how to predict reactions from adults when they've done something. So when you watch characters that don't follow instinctive motivations or reactions, they become hard to relate to - just caricatures.
It's like watching a football game where nobody follows the rules - how do you tell who's winning? Would you still care?
Same with a story. If the characters are believable and the situations realistic, the story becomes so much more impactful, the horror so much more potent. The struggle to survive becomes real. But when things just "happen", after a while, the tension is gone.
One of the best examples of consistent writing is The Thing (John Carpenter). They set the rules, and the story unfolds within those rules. The tension is real because we know exactly what's at stake.
Was I expecting too much? Or was this movie just meant to be a popcorn "summer blockbuster alien movie with guns"?