although you can still create a reaction that killed the person who used it. You can also create a reaction that creates no products (except boiling syndrome stone if you don't use Dfhack) and only effects the individual of a certain caste. So if you don't have the indivdual of a certain caste, it will just end up doing nothing. You can also make a reaction that affects animals by making it so that it temporarily gives the worker the ability to perform an interaction on the wanted animal and if the worker doesn't see the wanted animal in x amount of time, it just ends up doing nothing. The main problems begin to come into play when you create a reaction that consumes items and is meant to affect an individual since it leaves the possibility of it being wasted if you activate it when there is no individual around. It also becomes a problem when you want to use the individual to create items, although you can do that with Dfhack.
Actually nvm, if you want to create an item using individuals. Make the reaction produce a syndrome stone that transforms the victim into a creature that has [ITEMCORPSE:whatever item you wanted]. then give the creature the tag [MAXAGE:0:0] and it will die instantly.
So in summary make reaction drop a syndrome stone that transforms the indivdual into a creature with the ITEMCORPSE tag then give the new creature [MAXAGE:0:0] and you will get your wanted item. This has the plus side of not being restricted to civ members only, you can make it so that worker just gets the ability to transform wanted livestock into item by using interactions.
but the simplest solution would be to create a reaction that creates a syndrome stone as well as the wanted item (this will kill the worker 99 times out of 100)
so I hope my answer was helpful. And yes the soloution with itemcorpse has been tested.
So Putnam, did I blow your mind
EDIT: If you ever plan on using dwarfhack, replace the syndrome stones with autosyndrome.