Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
occīsĭo, ōnis, f. [1. occīdo], a massacre, slaughter, murder (class. but rare, except in eccl. Lat.; sometimes interchanged in the MSS. with occidio): si caedes et occisio facta non erit, Cic. Caecin. 14, 41: parentis, id. Inv. 1, 26, 37; App. M. 6, p. 184; Aur. Vict. Vir. Ill. 14: aestimati sumus sicut oves occisionis, Vulg. Psa. 43, 21: gladium ad occisionem, id. Jer. 15, 3.