Example: troll ghoul is attacking an wholly astral opponent with an unarmed blow. Because he is attacking somebody in astral space, he should have to use astral combat+willpower, causing Charisma/2 damage. So far so good. But what if this troll has killing hands as an adept power? This power is usable in astral combat, but for what reason? In astral combat you can choose if you want to do physical or mental damage anyway. There is also no ItnW in astral space and it doesn´t negate mystic armor. That you can take your close combat skill instead of your astral combat skill is also no option, because the skill of astral combat covers the unique ways of damaging opponents that can use the astral movement options. So might this power possibly help you to cause Str/2 damage? What exactly does it do?

no, a ghoul can always hit an astral thingy, it beeing a Dual natured creature.

a Metahuman Adept can only hit an astral thingy IF he has purchased the Killing Hand power.

The attack is still Willpower + Astral Combat with Cha/2 base damage in both cases.
The ghoul (vampire, dragon, whatever dual natured thing) doesn't need a power whereas a metahuman adept do.

Looks like it does nothing...

Maybe the line "Killing hands can also be used in astral combat" is a leftover from previous editions?
IIRC in SR3 you could buy Killing Hands in different levels giving you the ability to do serious or even deadly damage right out of the box. The above statement would than indicate that this ability also includes astral combat damage.

-CJ

Killing hands alone does not let you hit wholly astral beings. You need access to astral space to do this. So an adept would need astral perception to engage in astral combat. If he can perceive astrally, he may attack astral beings, but Killings Hands is no prerequisite for astral combat.

-CJ

No. Dual-natured characters (including those with astral perception) inflict standard astral damage (CHA/2) when attacking purely astral opponents. Killing Hands, Natural Weapon, Weapon Foci and other similar abilities and weapons lets them use their physical weapon (and linked physical attribute) to determine damage instead. Just running up and trying to punch an astral form with your bare fists doesn't; it's always going to be Charisma/2.

Could you please give me a reference for this for my GM? If this troll is a PC he is no "critter" and thus there is no official opinion i could find.

This is how I play it in my game, which is the way I understood the rules : the only weapons that can affect an astral being in the astral are either magical or your basic astral strength (aka Charisma which is divided by 2). That being said, a weapon foci, a dual-natured Natural Weapon power, and killing hands meet this criteria. Killing Hands, by definition, is a magical weapon which also allow the adept to choose between P or S damage.

So to answer your question, the troll is able to use Killing Hands because it is a magical weapon that has an astral presence, similar to a weapon foci. The adept is also the deciding which type of damage he'll do to the astral being (however, I play my goons using a single damage track, but that's another story/topic).

Here's something from the natural weapon power:


It's talking about "the critter", but I don't see why the power should work differently for a PC. As your troll ghoul already has the natural weapon power, he won't need Killing Hands at all... Natural Weapon will even let him use his normal Unarmed Combat skill instead or Astral Combat.

-CJ

Ah, great. Thank you. Maybe this will help.