What do we call an organism, typically a parasite, that ultimately kills its host?

The correct term for an organism that ultimately kills its host, especially in the context of a parasitic relationship, is a parasitoid. Parasitoids typically develop on or within a host organism, leading to the host's death, usually after the parasitoid has reached maturity. This distinguishes them from other types of parasites, which may rely on their host for resources but typically do not lead to the host’s death.

Pathogens, in general, refer to organisms that cause disease but do not necessarily have to kill their hosts. Nematodes, while they can be parasitic, do not inherently kill their hosts and can also include free-living species. Beneficial insects serve a positive role in the ecosystem, aiding in processes such as pollination or pest control, and do not exhibit the harmful traits associated with parasitoids. Understanding these distinctions helps in recognizing the ecological roles of various organisms and their impacts on the environment and agriculture.