Other plot points
Their motivations are relatively simple. One of the great revelations that happens in the next part of the story (possibly a second book, considering what I have planned) is that humanity once ruled the universe. Humanity is not native to earth, though Neanderthals are closely related to homo-sapiens. However, the great human empire fell into despair after several of their key worlds where hit with supernovas and completely destroyed. At this point, many remaining humans fled to Earth. However, many others chose to "evolve." Much like in "2001, A Space Odyssey" around half the remaining humans in the core world's chose to become energy beings. This lead to their far reaching worlds (the Azurans are one such example) to forget their origins, or in the case of the Invaders, to worship them.
That wouldn't make much sense, if not for an AI that went insane after it lost contact with the rest of empire. The AI led the Invaders to believe they were the destined children of the empire, and the remnants of the empire were their rightful inheritance. This causes a thousand year crusade to check up on every world the AI has in its databanks, conquering every world they come across and absorbing their peoples, their cultures, and their technology.
This led them to the Blue System, and the Azurans. When they first glimpsed the Azurans, they thought them to be devils and demons, as none of the other planets inhabitants had skin colors such as theirs (most of them had skin colors like Earthlings, including the Invaders themselves). When they posed the question of what the Azurans where to the AI, it read their fears and confirmed them, causing the systematic slaughter of an entire race.