Well, what would you find more frightening; a man using a gun to shoot people, or that gun deciding for itself who it will shoot?
The thing is, we see X and reploids as thinking beings, so of course we know and understand that they can hurt others. But humans in that time period see robots as TOOLS, designed by function to be used by humans. The idea of those tools choosing what to do by their own accord is, by definition, frightening because they're not supposed to DO that. The idea of them willingly choosing to HURT people is even worse.
There's dozens of movies where smart weapons decide to run amok, or where appliances or cars try to kill people. There's a reason those are horror/thriller movies.