Scott's own words.

I have the 6 disc Blu-ray Alien Anthology and it just happens that I was watching Alien 1 last night (again) with the Ridley Scott commentary. Following are his exact words as Dallas and his 2 crew members enter the Space Jockey chamber; I always wanted to go back and make an Alien V or VI where we find out where they come from and we go there and answer the question "Who are they?" Mars is too close so they can't be gods of war. But the theory was, in my head, was this was an aircraft carrier, a battlewagon of a civilisation and the eggs were cargo, which were essentially weapons. Sort of like a large form of bacteriological stoke. Biomechanoid warfare. The Space Jockey, I've always thought was the driver of the craft who is now after many ages. But clearly from here, this is where the transmission would emanate from, probably in an automatic transmission. So this creature, obviously, had experienced maybe one of the eggs had been disturbed and creature had got out and had attacked the rest of the crew. Don't ask me where they got to. But he's pretty gruesome. But let's say he was part of the civilization he came from and now melded into his seat. End of Quote I thought it interesting that he says the transmission was probably automatic which denies the assumed benign motives of the SJ to warn others away. Someone in another post said that warning others away to not suffer the same fate didn't quite gel with the nature of the cargo.