To create, first one must destroy

Judging by the fact that he is a mad Nazi robot he would probably say because you have to erase the imperfection of unworthy creations/creators.

But because he is mad, he could response that the whole thing is intended as a wedding gift for the newly weds from the picture.

That is a BIG Question!

Regarding David it depends on how much he could Gauge was the Reason from what he had learned/assumed from LV-223 and we dont know how much he knew as its a case of how much information could he gain from the Computer Systems he accessed and the Engineer Writing on the Walls/Doors etc.

In Reality  From Destruction does Rise Life, some Events have been Necessary for others to happen...  The Death of a STAR can Spell Destruction for its Solar System but then also kick start NEW Beginning over Many Millions if Years/Billions.

If the Dinosaurs were never killed off 65 Millions years ago, would Mankind had been here today?

We can look at it in a Biblical Sense too, GOD had to Destroy the Earth with the Deluge so that he could Allow for a Fresh Start...

We also have the Natural Order of things.. Evolution, Survival of the Fittest... Some of Evolution spelt the end for certain Life... which allowed other Life to Strive!

Its really open to either as in terms to what David would have said if you asked WHY?

I think RS comments and parts of Alien Covenant give us the Answer....   When the Engineers Deem a World/Creation to be a Failure or a Threat...  in order for them to begin with something NEW they have to Destroy what was there before.

I don't think David could or would give a logical answer/justifiable and might just be coy and say "Because I can".

I suppose, he could say, being only partly smug, that he just let the natural take place - evolution. The survival of the fittest.

It is only natural for smart specie to outmaneuver dumber one out, especially if the later is a survival risk. Where are the denisovan, neanderthal and australopithecus? Dead and assimilated.

BioDegradable That is a great point! Purity.

However, the Engineer seemed both smarter and stronger when it ripped David's head off.

The "To create, first one must destroy"

Was Davids Response to when Dr Shaw asked WHY, after David informed them the Ship was scheduled to go to Earth before things went to POT!

So we know at least David knows what the Bio-Weapon can do, and we know he knows the ship intended to go to Earth, we just dont know if David has any idea as to WHY.

So a lot depends on what David learns about the LV-223 Experiments, and what he learns about WHY these Engineers create life on Worlds....  but these are things the viewer is never shown... we can only assume what David found out.

At the Basics.... he discovers the Black Goo can Manipulate/Evolve Organic Matter into something NEW as we see with the Hammerpedes and Fifield... and so its easy to speculate that the GOO Destroys a Organism, and from this we get a Evolution.   And so indeed from Destruction would come Creation...

We could speculate or David could that the Bombardment on Earth would Destroy all Life and from its Death would arise NEW Life that is based on the DNA within the Black Goo.

This theme is repeated in the Franchise... a Xenomorph requires a Host and its Birth signals the Death of the Host. "To create, first one must destroy"

This is what David would know the Bio-Weapon would do... but WHY... thats the Big Question... at the Basics David would know these Engineers have not been back to Earth, and something triggered them to want to Destroy Mankind or at least Upgrade us... and in this Complex David saw a Devotion to the Experiments, the Mural, the Fresco and David would see that Sacrifice was needed to Give LIFE to Organisms depicted... and these Engineers seemed to HOLD these Organisms in High Regard... yet had abandoned Mankind.

So at very least David would have concluded the Engineers had created something more PERFECT and deemed to Evolve Life on Earth to this more PERFECT abomination.

This NO-DOUBT is where his "To create, first one must destroy" reply came from....

If Dr Shaw asked WHY, then his response would depend on IF the above is the only information or conclusion he has.

Maybe his reply could quote Mary Shelley

"Man's yesterday may ne'er be like his morrow;
Nought may endure but mutability!”  

or

"Nothing is so painful to the human mind as a great and sudden change.”

Where he would then Smile and Give no further Explanation, for IF Dr Shaw understood Mary Shelley and could peace together what she has seen... she would get her Answer...

Evolution/Change is a Necessity