The only free and comprehensive online etymological dictionary of the Spanish language
matar (Verb) "to kill"
10th cent. Of unknown origin. There are three plausible theories. The youngest theory is an origin in Arabic mata "he has died." The oldest theory is that matar derives from Latin mactare "to slaughter" through a hypothetical Vulgar Latin word *mattare. A final theory connects it to Latin mattus "stupified" via a Vulgar Latin word also reconstructed as *mattare. See Dworkin (2012) for a full discussion.
Indo-European
Romance
Asturian matar, Portuguese matar, Galician matar, Catalan matar, Italian mattare

"Primary verbs of Arabic origin are rare in Spanish: achacar 'to blame, criticize', atamar 'to finish', halagar 'to flatter', recamar 'to embroider' atracar 'to bring a ship alongside the dock', zahorar 'to eat late, have a feast'. The verb nicar 'to copulate' turns up once in the Cancionero de Baena... Does the infrequency in the recipient language of primary verbs borrowed from Arabic constitute sufficient grounds to definitively reject the proposed Arabic etymology of Sp./Ptg. matar 'to kill'?" ~ S. Dworkin, A History of the Spanish Lexicon (2012)