Those constructs and demons were specifically marked as also being humanoid in the code, that's why it works on them too.

Anyway, found a humanoid slayer? That's awesome - enjoy the higher damage and try not to get too overconfident, instead of feeling it is too good. Not everything has to be balanced.

Yeah, it's the best slaying power but there's nothing wrong about there being a best slaying power. Slaying ammo is a very limited resource anyway (for non-scummy players at least) so you need to treasure your humanoid slaying ammo, it's not like you can go through the game slaying everyone left and right.

Obviously there is a game mechanic that makes it work that way but I don't understand the reasoning behind it. I'm not thinking so much about slaying ammo as about melee artifacts. I think ammo is ok the way it is.

There are like four artifacts that slay humanoids: Skullcrusher, Kinslayer, Executor and Vanquisher. Taking away humanoid slaying from Skullcrusher and Kinslayer will make them complete garbage, Executor doesn't give a crap about humanoids because it already slays demons (that's what is really important) and Vanquisher is really situational.

I'm not saying that it should be taken away, just that it shouldn't work on constructs (and maybe demons, but constructs is what bothers me).

I think you're looking the wrong way manseman.
Delve for a moment into the idea of humanoid slaying.
By definition humanoids would need to have two legs, two arms, a head and a torso.
The weapons with their critical hits against such opponents cause damage to those crucial areas that connect each section (or so I imagine).
It's only natural that humanoid slaying will have an effect against any creature that is built based on those principles.

Instead of removing the feature that makes humanoid slaying work on constructs, ask yourself if all constructs have to be humanoid?
Perhaps there's a reason they are but there could be plenty reasons to make them different.
After all they are constructed beings.
They may very well look like a walking table.
It is that issue you should address - modify the in-game conception of constructs to be non-humanoid.
Then there will be no problems with humanoid slayers covering such a large portion of possible enemies.

Well, perhaps there are other game mechanics that depend on constructs being regarded as humanoids in some aspect that you don't know about where it makes more sense. But sure, if the same thing can be achieved by not treating constructs as humanoids with no side effects it wouldn't make any practical difference.

I don't think anyone said that any artifacts should have their slaying properties changed. You would be making a change to the actors, not the equipable. We are maybe talking about setting is_humanoid attribute to false on some group of actors.

EDIT:
Yes, anon, I think we all suspected the current behavior might have something to do with the way the game was coded.