bestiality (n.)

late 14c., "belonging to a beast," c. 1400, "having the qualities of a beast," from Old French bestial (13c.) "relating to animals; beast-like, stupid, foolish, brutal" and directly from Latin bestialis "like a beast," from bestia (see beast).

The sense of "below the dignity of a human" in English is from c. 1400, and often is unjust to beasts. When the beast of the Book of Revelation was meant, the adjectival form bestian (1650s) sometimes was used.

word-forming element making abstract nouns from adjectives and meaning "condition or quality of being ______," from Middle English -ite, from Old French -ete (Modern French -ité) and directly from Latin -itatem (nominative -itas), suffix denoting state or condition, composed of -i- (from the stem or else a connective) + the common abstract suffix -tas (see -ty (2)).

Roughly, the word in -ity usually means the quality of being what the adjective describes, or concretely an instance of the quality, or collectively all the instances; & the word in -ism means the disposition, or collectively all those who feel it. [Fowler]