This verb was mentioned in several places with different meanings. In Late Notes on Verbs Tolkien gave 1st-sg aorist and past forms of this verb along side the more “intensive” verb nahta-:

[The ta-formative element] t was also used as (originally) an intensive or differentiator as in √NDAK, hew, ndakta, slay, to[?] Q nakin, nanke/nahtan, nakante, slay, slew (PE22/156).

Here, it seems Tolkien was describing two verbs: a basic verb nac- “hew” derived directly from the root √NDAK and a derived verb nahta- “slay” from a ta-formative augmentation of the root. The verb form nakin was mentioned in passing in Vinyar Tengwar 49 as nakin “I hew, cut” but without an indication of what the actual source was (VT49/24).

Conceptual Development: The verb ᴹQ. nak- also appeared in various forms in the Quenya Verbal System from 1948, generally with the gloss “kill” but in one place with the gloss “hate” (PE22/112, 120, 123). In Primitive Quendian Structure from the 1930s, Tolkien gave nakuvan as the modern Quenya form of ancient ᴹ✶ndăkŭbā̆nyē “I will slay” (PE21/65 and note #13). The change in meaning for Quenya nak- seems to reflect a conceptual shift in the primitive root: in The Etymologies of the 1930s ᴹ√NDAK was glossed “slay” (Ety/NDAK), in the Outline of Phonology of the 1950s it was glossed “hew, slay” (PE19/91), and in the 1960s only “hew” (PE22/156, see above).

Neo-Quenya: For purposes of Neo-Quenya, I would assign nac- the meanings “hew, cut”, and for “slay” I would use nahta-. Furthermore, I would assume nac- originally applied to the result of hewing a thing = “hew [into], cut”, as opposed to mac- for a hewing motion. I would further assume the meaning “hew [into], cut” was influenced by the homonym [ᴹQ.] nac-² “bite” of different primitive origin (Ety/NAK), and I think the “bite” meaning of nac- is more common since mac- can be used for “hew” if things are ambiguous.