I think the low ratings are largely because the film is just so depressing (spoilers)

Rating: 8/10 (17 May 2017)

I don't think this is a poor film, I actually think it's one of the better of the Alien franchises. But it's just very, very negative. Evil wins.
It's based around the idea of a robot (or 'synthetic') version of 'Kurtz' (David) from Conrad's Heart of Darkness, going 'rogue' on an isolated planet. Nothing wrong with this idea, but one really expects towards the end, that, like the character also based on Kurtz in Apocalypse Now, good might eventually right the wrongs of 'evil' and the rogue Kurtz-style robot might be knocked off or lose, but in this film that doesn't happen.
Not only does just about everybody get knocked off by the alien pathogen in typically gruesome fashion, the 'good' robot (Walter) gets knocked off by the rogue robot (David), and this bad robot then manages to take over control of the rescuing ship, as well as 2000 people in deep sleep, and the entire mission, (which is to colonize another planet), signaling his intent to infect them with the alien pathogen whilst they are in deep sleep and helpless. One couldn't think of a worse outcome, really.
One leaves the theatre feeling that 'evil' has won. I don't think I've ever seen as negative a movie as this, and I think people are so deflated by it that they vote that way.
The cinematography is outstanding, and the story makes some general sense, (except where the crew systematically fail to realize their danger on multiple occasions, for example; after a crew member sees one of the aliens murder a crew member in front of a non-protesting David, that crew member then completely trusts David when he shows him a vault full of alien experimentation, and so gets murdered by another one himself. I don't think so. Also, a superior robot is murdered in close combat not once, but twice, by a less superior one; now that's stretching things a bit. And how can a trained crew land on a remote planet with no measures or thoughts of protection from potential viruses or other pathogens? Wouldn't want to catch a cold with no immunity).
I would have altered the script to something slightly more palatable; one expects the next in the series to restore some balance in that respect, you can't have evil always 'win' and expect people to continue to like it. I also don't think people will want to see 2000 people eaten alive in deep sleep by aliens, which is logically where this might be going.