Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
perpĕtrātor, ōris, m. [perpetro], a performer, committer, perpetrator (post-class.): peccati, Aug. Civ. Dei, 20, 1: caedis infaustae, Sid. Ep. 8, 6.