arms race ?

I think you are right, to a degree. It's possible that the engineers are at war. The original faction had already developed the xeno as we know it, and the new faction was trying to produce a new and better xeno process to win the war, but it was too infectious and wiped out every single facility on that planet and every single Engineer except the one Shaw's team finds. If the war started 2000 years ago, the new faction might have wanted to destroy earth to cut off a possible supply line.

maybe the faction the made us had become so peaceful the had to make use to wage war . and they warned of the other faction , i.e. the cave pictures , that were not " an invitation " but a warning .