Movies are a multi million dollar investments
Every little thing in them is chosen, debated, designed, scrapped, redesigned, shot, edited a thousand different times.
The assistant's character wasn't organic, she was written that way, the director then decided to take that amount of time to kill her. For the most part its not that she died that is the problem. She is a bit character in a monster movie. She's not a star character and she isn't a kid, so the audience is aware that she is probably dead the first time we see her.
What people are uncomfortable with is the brutality of her death and what this brutality represents. She didn't just die, she was tortured for a good few shots before being killed. These shots cost millions of dollars. They deserve to be analysed and dissected and their context to the rest of the movie and to the society of the time.
We see that she is pre-occupied with work or whatever it was, we see that her pre-occupation means she loses the children, we see that she isn't really that bothered by this aside from what it means about her job, we see her get brutally killed.
The people responding that MEN DIE TOO are missing the point wilfully or not. Most of the guys who die in the movies either die heroically or are villains or are not even characters in the first place. For instance let us consider the death of the assistant compared with the death of Eddie in Lost World. Eddie dies heroically, he sacrifices his life to save the main characters. What message does that send? Now what message does it send to kill someone for the crime of looking at her phone?
TLDR: The people coming in to the thread defending this movie, or really any movie by saying it is just a dumb popcorn movie and we should all lighten up are not actually doing the movie a service at all. Movies deserve to be looked at with a critical eye. Whether they are successful or not too much time and effort went into them to be dismissed as dumb popcorn.