There has been alot of debate regarding what is actually considered a break in the honour code & what isn't, this should be quite interesting.
As far as Billy was concearned, he had made a last stand. His squadmates had been killed, & he had made a strong but silent challenge to face the predator one on one. It would be considered cowardly & dishonourable to simply walk away from the challenge, so the predator took him on & killed him. While one man with a knife (it wasn't a machete, despite what alot of people have said) may not seem like a great threat to a predator, the principle of the challenge & the hostility & willingness of the prey to fight meant he was still a worthy kill. How the predators can identify certain objects in their hazy-looking vision modes has always been a complete mystery though.
Both men who were gunned down, Blaine & Mac, were also actively searching for the predator at the time they were killed. Blaine had his minigun armed & was searching for Hawkins' killer when he was taken out, & Mac was planning to ambush the predator after Dillon had flushed him from his hiding spot. The plan obviously backfired. Both Mac & Blaine had intentions to track & kill the predator, & were armed with significant weaponry to do so, thus they were deemed honourable prey. Poncho though was a different story. At the time of his death, caused by a plasmacaster bolt to the head, he was extremely injured & could barely stand due to being hit with a log trap, which was originally being used as part of the team's plan to take the predator down. At the time of his death, he wasn't very able bodied, but still brandished a weapon in his hand with hostile intentions towards the predator, as he had all along, so was still considered an honourable kill.
As far as the surface camp in AVP goes, the scene was probably intended merely to announce the arrival of the predators on a strong & powerful note as they decimated their prey. While they were there for the aliens, the guards at the surface still posed a reasonable threat, & could have potentially called in backup if they had recieved any sort of transmission informing them of the events that were taking place down in the pyramid. They were all armed with assault rifles, & more than willing & able to defend themselves. They weren't actually considered as prey though, as not a single trophy was taken. The predators were ultimately there for the aliens, to complete the blooding ritual, & nothing more. According to speculation, predators aren't allowed to hunt humans for trophies until they have killed an alien & earned the right to become blooded.
Xenomorphs are considered extremely dangerous prey, their hostility & natural killing ability means they are considered worthy prey. They are physically just as powerful as a predator, & are extremely hostile to anything except for their own kind. Considering them dishonourable prey simply because they do not brandish technological weapons is the equivalent of a big game hunter turning his back on a lion because it was not armed with a gun or a knife. The xenomorphs also did attack first, contrary to your statement Crow. Chopper was impaled through the abdomen & had Grid's inner jaw slammed through his skull in a completely unprovoked assault.
Anna & Lex on the other hand did not present any sort of genuine threat to the predators, nor did they ever make an attempt to attack or kill one of them. Prior to the fight between Celtic & Grid though, Chopper was ready to kill Lex, as she had made an attempt to reach for a large ice pick, presumably to attack Celtic with. The situation in one of the hallways where Scar had threatened Lex until she turned over the plasmacaster was more than likely intended as a bluff. All he wanted was his weapon, & using intimidation as the means to get it seemed like the most effective solution. Although why he aimed his caster at her afterwards is beyond me. I'm assuming it was intended more of an "introduction" of the plasmacaster into the movie. Anna on the other hand was too terrified due to the events occurring around her to try & act against the predator. She gave slight aid to Dutch's team in certain situations, but beyond that, she posed no threat whatsoever to the predator.
Leona from Predator 2 had her life spared due to her pregnancy. She had hostile intentions towards the predator, although killing her would mean killing a defenceless child also, which would be considered an extreme dishonour. Due to this, & the fact that the predator had her at his complete mercy, meaning she was no longer able to defend herself, her life was spared. As far as the queen is concearned, the only time we have seen a predator pitted against one was after she had already detached from her ovipositor & was currently incapable of laying any more eggs, thus meaning she was considered fair game.
Judging by all of this, the main principles of the honour code in my opinion seem to be:
Prey which poses any significant threat, or has a strong willingness to fight is considered fair game. Prey armed with weapons which have the potential to be harmful is considered fair game, whether the weapon is technological or natural is of little concearn. Pregnant females are not to be killed under any circumstances unless they pose a severe & critical threat. Simply subduing or incapacitating them is the preferable option if necessary.
Using underhanded tactics against prey is considered a dishonour. If the situation strongly places the odds in the predator's favour, try to even out the playing field by using less formidable weapons, if needs be, even expose your own weaknesses to give less able prey a fighting chance. Killing other hunters is strictly forbidden, & is punishable by death. An exception can be made in the case of bad blooded yautja, which should be killed on sight to bring an end to their shame & dishonour.
Allowing the secrets of the yautja species to be uncovered & become widespread knowledge amongst the more intelligent prey is considered a great dishonour. If needs be, initiate the self destruct function in the event that you are likely to be captured or killed to prevent the technological secrets & existence of the species from being uncovered. In the event that the secrets fall into the hands of those who would willingly use them against us, banishment & exile would be fitting punishments if the one who allowed this dishonour were ever to return.