I think Characters as Thoughts_Dreams mentions a lot, is certainly key... Well handled Characters we can connect with are key to any fear/horror.
If Characters mean nothing to us, then even revealing the Xenomorph a few times like in ALIEN would just amount to a few Jump Scares.... For us to have real tension we need to believe in the Plight of the Characters.
I think Context is key, and pushing boundaries... pulling something a bit more risky/shocking..
RS said Alien Covenant will Scare the $"%£$^ out of us.... Sadly for how Gruesome Ledwoods Death was.... the impact was shock value only... because well Ledwood was basically just another Star Trek No Named Away Team Member.
If this scene happened after, to a Character we could connect a bit more with then the Impact would be much greater.
Can you imagine if the Back Burster Scene happened in Alien 3 to NEWT?
Whats more, is i was expecting Alien Covenant, due to it having couples, and seeing those Baby Looking Aliens in Davids Workshop prior to any Trailers landing... i was pondering a idea and a wish.... That what if Black Goo infected a Pregnant Female, who then had a Love Scene of Sorts... and we got a BELLY-BURSTER... How Crazy would that be?
Not to worry thats what i have in store for my ideas for a AC sequel... ;)
The biggest task is putting David in a situation where you can feel for his loss/situation but doing that means making us think of him more than a ROBOT...
This again is a point Thoughts_Dreams brings up a bit, and its valid... its really hard to get us to Feel any Fear or Sorrow for a Robot in a movie.... i mean we was not really shocked (well i was not) or effected by Bishop being Torn into TWO by the Queen.
If Bishop was portrayed as the Bad Guy, then we would feel even less Horror at such a Scene...
I think Families hit home with many people..... putting them in a situation of horrific events like Alien did, Alien 3 or say The Thing could be key to Ramp up the Horror....
A Alien Covenant Sequel certainly gives opportunity to exploit this.. especially if we visit these Colonists a few years after they arrive on Origae-6 ;)