Pretty sure the Deacon was a "chestburster" of sorts.
Accelerant gives rise to lots of nasty critters.
Pretty much an infinite variety.
Some of them killed the engineers in LV223
and some even chestbursted out of them.
Out of all the things in Prometheus... this is the easiest one to explain.
The film gives you all the hints.
And as to why engineers created humans?
Prometheus also gives you that answer.. with David's question:
"why was i created?" "Because we could."
Why did the engineers hated humans?
The film also gives you that answer...
with the Weyland and Vickers relationship
being that of a parent who hates his child,
It's a neat paralell that invites the audience to explore a big piece
of the lore by questioning human relationships.
And then Weyland is killed by his own creator.. while the daughter he hates watches from afar. A hateful parent killing a child who hates its own child.
I dont think Prometheus is a good film
trying to do some interesting things with its themes. It's a very pretty mess.