- Don't spawn kill
In Aliens vs. Predator 2, as opposed to most games, there is a period of time shortly after spawn where the spawning player is invincible (he appears as translucent in the game). Once the player starts moving, or a period of time has elapsed, a chime is emitted and the player solidifies and is then able to take damage. It's an unfortunate "feature" in the game that players seem to appear in the game world before they themselves respawn, meaning that while they're waiting to respawn other players can see them in their new spawn position before they're there. This leads to the temptation of players attacking spawning players exactly while they're spawning, rather than giving them time to enter the game world. This is generally considered very unsportsmanlike, especially in a game where players appear at their spawn locations early and have an invincibility period.
If you see someone spawning, be nice. Give them a pass.
- Don't take advantage of the magnanimous
The flip side of any guideline about spawn killing is not to take advantage of it. If you're spawning and see someone walk past, deliberately not attacking you because you're spawning, then don't turn around and immediately attack them while their back is turned; head off in another direction and you'll both live to fight each other later.
If you're spawning and see someone choose not to attack, take a pass on them as well.
- Don't corpse camp (lifecycle)
In lifecycle games, the aliens start out as facehuggers and must facehug hosts (humans or Predators), escape as a chestburster, and survive for a period of time until they can transform into a full-fledged, adult Alien. As a chestburster, they are the most vulnerable; they have reduced mobility, cannot wallwalk, and although they have teeth, are easily killed. "Corpse camping" is the process of humans or Predators waiting over an impregnated body for the chestburster to appear, and then immediately killing it. This is considered fairly unsportsmanlike; it is really the lifecycle equivalent of spawn killing. It is, of course, totally legitimate to kill a chestburster that you encounter, and even to shoot an emerging chestburster as you're passing through. But it's somewhat unfair to intently do nothing but wait for a chestburster to arrive, particularly when there are only a few Aliens playing, in the same way it's considered unsportsmanlike to camp spawn points for your enemies to appear.
The one positive side is that corpse campers tend to not be paying attention to their surroundings, and make easy targets.