This is what you get when the writer of the story is the editor of the story and the director and the producer. There's no one to tell him, "hey, this is not good. You need to fix that."
All around the movie is surprisingly good. The acting was believable, the characters and characterizations are good, the sets are good, the continuity was accurate, the music was extremely good, and the special effects were adequate to the job. Many bits and pieces are lifted from movies like Close Encounters and Signs, but that's ok too.
The first 3/4s of the movie are edge of the seat chills and thrills, but then the story shifts in a totally unexpected direction. This is considered "the unique twist." The 'twist' is something that modern scriptwriters seem desperate to pull off and almost none of them actually do it. The Sixth Sense may be the last movie where it really worked. In this particular movie, the writer/editor/director/producer throws in the monkey wrench without the slightest attempt to justify it except to offer a lame, "I don't know. We don't understand it. We may never understand it." It's Aliens, so sure, how could we understand it? But that's just not good enough. The motivation fails. Why the Aliens do what they do makes no sense unless you just accept "well, they are aliens, so their motivation is alien." That may be fine for people who shut off their brain to "enjoy the movie," but for people who enjoy the movie by thinking about it, it doesn't work.
The plot twist does not ruin the movie but it leaves an unsatisfied feeling that a good storyteller would not leave. If you don't mind being let down by the ending and being left scratching your head trying to make the nonsensical reasoning, it's worth a watch. There are a lot worse movies out there.
