It's an honour thing in the first two AvP books and it comes over very much as an honour thing in P1 and P2. In the first of the books the alien embryos are supposed to be implanted in non-sentient animals on a deserted planet; the Yautja don't realise that humans are in the process of colonising the planet and when the Yautja leader realises that the xenomorphs have got into non-combatant human settlers he is horrified. When young and undisciplined Predators start killing unarmed humans he executes them. The AvP movie deviated very much from previously established canon - although the Scar/Alexa relationship does seem to be based very much on the Dachande/Noguchi aka Dah'tou-di relationship.

In the third AvP book the honour principle slips - seemingly because of Yautja politics but more likely just because it's a different writer.