But not a 'Weapon of Mass Destruction', I think!

We can even assume that they came by once and then in the past (prior to their own problems) why would they kill earth 2000 years ago? Did Christ scare them? :p

The Jesus of the Gospels is a literary creation. Christianity was a Mystery cult with just a few thousand members until the third century CE. There was a lot more happening in the world. The paucity of imagination here is staggering. :(

the missions to earth first established contact, took some people back to LV 223. After a while, the now altered humans are bi ggrey space jockeys, and like in the old days of lsavery some slaves are forced to go collect more slaves for the master... but here it may not just be about the galactic equivalent of picking cotton, but also Igor bringing bodies for Dr. Frankenstein. They may be slave traders and war mongers. Drop a few space jockeys poisoned with black goo for your enemies to capture, and afew hours later, a Xenomorph is causing trouble for your enemies.

And nysalor... weren't some of the drawings from LESS than 2000 years ago?