My theory of horror films, discussed at length in my book, is that the primary purpose of horror films is to present a deformed and destructive being. This is obvious in a movie like Frankenstein, where there is a being (here, the Frankenstein monster) who is deformed and destructive and is clearly the focus of the story. But there are numerous cases on the edges of horror where the categorization is not so clear. Such a case is Cronos (1993).
Cronos is widely recognized as horror, both by those who like it, such as Bloody Disgusting, and those who don't, such as PelleCreepy. And it does have a plot that sort of sounds like horror: old man Jesus Gris finds ancient device that confers immortality on its user at the price of transforming him into a vampire. As Gris becomes a vampire through deforming makeup effects, he clearly qualifies as a deformed being. But he is not obviously destructive.
Gris does not kill to get blood; he scavenges blood where he can find it, from a nosebleed or wound. He is kind and generally nonviolent. However, it is clear that he is tempted to get blood more openly--he almost takes it from his beloved granddaughter, but holds back. And he is destroying himself through his addiction to the device; his visible suffering steadily increases as his vampirism gets worse. He is, therefore, destructive in two ways: potentially destructive with respect to others and actually destructive with respect to himself. He is a deformed destructive being whose presentation is the purpose of the movie. That makes Cronos a horror movie, although a peculiarly gentle and offbeat horror movie, in which the monster is almost preternaturally nice.
Deformed and Destructive Beings: The Purpose of Horror Films