Xenomorph mural in the Ampuel chamber

Sorry if all these questions have been asked before but... What is the relevance of the xenpmorph mural in the Ampuel chamber? Could it mean that the Engineers have some previous knowledge of the Xeno or that they maybe treat them as some sort of mythical beast that they worship? Is there a general consensous regarding the reason the Engineer attacked David and the humans? and does anyone have any ideas on what David said to him? The cave paintings on Earth also confused me a little. Why would the alien race leave maps directing us to their military base and not their home planet? I'd love to hear some more opinions on this.