Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
murmŭrātor, ōris, m. [murmuro], a murmurer (post-class.): summussi murmuratores, Paul. ex Fest. p. 299 Müll.: adversus Deum murmuratores, Aug. Retract. 2, 20; Vulg. Judae, 16.