Here is what I think. It is poetic justice, the engineers wanted to live forever and lost sexual reproduction, the purpose of life, by getting it. The engineers are evolutionary completion, they gave themselves eternal life with their science. This led to them also becomming sterile, their appearance is symbolically sterile, they have no hair, no wrinkles, they're completely pale and clean. But they're not invulnerable, so they're slowly dying out and need to find a way to reproduce again. So they made the black goo so they could evolve back to a specie that can produce offspring, using their own dna to give life to other planets. They learned the hard way that death was the price for life, that is why the engineer ritually kills himself to create life on earth. In some retellings of Noah's Ark the reason god caused the great flood was because angels had sex with humans and made half-angels. David and Walter are robots who live forever, they are also sterile. The engineer was upset with Weyland because he wanted to live forever and David represented that the humans were becomming sterile themselves. Wanting and trying to live forever is blasphemy to them. He killed Weyland and David to save them from themselves