David didn't create the first Xenomorphs.
David managed to replicate something the Engineers had already done. Create the perfect organism. Alien has an Acient Engineer ship with eggs in the cargo, and they seem to be placed in order too.
Well there is imo a difference here, the engineers did indeed created something perfect, or lets say they created something,
but im not sure off what the essence was of that creation, was it to serve them, or was it truly a weapon?
One thing is for sure that David created the perfect killing machine, im not so sure the engineers were creating that, or tried to create that.
Maybe a far stretch of what im saying but, its like with our nuke power, we can give countries energy from it for years and years,
yet unstable it could wipe us out, and being fitted into a bomb/rocket it becomes also very dangerous on a war scale.
The murals also show the engineers holding down some type of creation, the face/head part is very hard to see in that mural,
so imo its not a xeno they are holding down.
But then if you jump forward to Alien and the derelict cargo hold they have those eggs, but those eggs are kept in stasis through a laser vibe thingy.
So they are aware of eggs and are able to keep them under control with something.
It could also be that the engineers created a controlable xenomorph, heck maybe even one that thinks for itself and follows their orders, but not one that is untamable and uncontrolable and only lives for the expansion of his "reich" David did just that,
he made an unstopable killing machine.
And it doesnt make sense if a race like the engineers creates creatures that they cant control, so like with the eggs in stasis i think the engineers also are able to control a xenomorph through something, and David just took away that safety pin.