Specicide

This is regardless of the method, such as hunting down all or most of the species until it is completely wiped out or incapable of procreation, using some form of magic, devicing a disease, contagion, virus or such that attacks the species, or some other more imaginative method.

Obviously not very good thing if you do this for your own species as that would very likely include yourself. Exclusion of the protagonist can be ignored for this if they're incapable of continuing the species' existence (e.g. if they're the last human).

Related:
* Genocide