I'm tired of editing the Grimm page, some I am just going to say here, that in Latin, decapitare is Vocative. If you are supplying Latin for a definition you use the Nominative. How can I tell? I am in Latin 1, and in Latin 1 you learn to decline. In the tunnels, Gregoric says that he used to have respect for the decapitarī. This is an incorrect form, it would be decapitarīs (Dative plural) but the writers are still implying that it is second declention because of the ī. There is no ending in the second declention that ends in e. That is in the third, and that is a little better, but still very wrong. The only other option is it being an infinnative, and i dont know about you, but it does not make much sense to just look at someone all evil like and say "to decapitate." They probably got it from Google translate. The difference is that the Vocative case is used for nouns of direct address (you who decapitate) and the Nominative is used for subjects (he who decapitates). Before you say "It does not matter," trust the certamen player, it does. Before you say "But Rosales said..." She translated it wrong. Any protests from other Latin scholars, express below, and then edit. Please.
When supplying people Latin it should always be nominative singular, unless specified otherwise. Trust me. When asking for deerivatives or the definition, my Latin teacher won't accept your answer if it is anything else, nor will moderators of a certamen match.