I really think it's an ego thing with Preds tbh, they hunt for the status and enjoyment that comes from killing deadly prey. A lot like big game hunters on Earth, really.
I think that was always the intention of the film-makers (the things characters are scripted to say seems to support this), that the Predator goes for those who it feels represent the greatest challenge to its skills as a hunter, a trophy it can show off to other Predators back home, or hang up on the wall as a souvenir.
Personally I like this idea. All the honour and samurai bullshit just pisses me off because it's got no foundation in the films. The predator just kills anything that presents a challenge to it other than an old guy who's terminally ill, a pregnant woman, and a kid with a fake gun.
It doesn't kill Weyland because he's going to die anyway, you can't take glory in killing a weak specimen, so there's no real point.
It doesn't kill the pregnant woman because it may go against some sort of code, like not killing pregnant females because it damages the population, or maybe because it's taking a life that is unable to defend itself in any way whatsoever, being barely sentient, if at all.
And it doesn't kill the kid because, well, apart from being a kid, the gun isn't even real.
Plus, the way the predators kills some of their foes don't seem very honourable to me. Like sitting in a tree while invisible and shooting them in the back.