I feel I should point out that alchemicals bodies can be salvaged for materials to make a new alchemical including the soul gem. Killing a foreign alchemical and looting the corpse is a cost effective if risky way of reducing the cost for your next alchemical construction if you can gurantee you don't lose any yourself.
Partially.
I think, though I won't swear to it, that a fair amount of the value of an Alchemical body is expended each time one is instantiated.
That's why a failed catalyzation is such a big deal, IIRC; you're never getting some of that back.
And of course, like
@Imrix pointed out, if people get the impression that you're killing foreign Alchemicals, they'll get a wee bit cross with you.
Including your own Alchemicals, quite possibly, given that Alchemicals from multiple nations do deal with each other over the course of multiple lifetimes and develop working relationships.
Do remember that Alchemicals are the closest thing to free agents in Autochtonia.
Loyalty is all that prevents an Alchemical from walking off the job and going to another nation; Kamak has apparently had it happen several times in it's history, and Claslat recently(in 2E) had an Orichalcum move to Nurad because she had a religious calling to help a nation in need.
You really don't want to erode their loyalties.
And that doesn't even include the possibility of the Adamants or Divine Ministers taking corrective action.
You can't guarantee it but I imagine nations with a military advantage will be interested in playing the odds. Doing it in peacetime would piss off the other nations and likely start a war but it is probably standard practice to loot alchemical corpses (yours or the enemies) whenever possible when you are already in a war. Similarly recovering your own alchemical's bodies cuts off most of the cost for a replacement so the real loss is in experience and charms.
And they do it to you. And the odd Apostate helps this process along.
And next time the Viator or the Palladium Worm shows up, the Eight Nations are significantly under-strength in Alchemicals.