My own take, and I think that it's fairly well backed up, is that we need to treat "Old Man Logan" for what is, a piece of art, (no questions of high/art low/art or otherwise), in the sense that it is a piece created in a large part by Mark Millar.

 In Millar, we can find our answer.  From the original "Ultimates" on, we can see a very distinct take on the Hulk that really isn't anything to do with attempting to fit something into canon---but more about Millar's interpretation as an artist.  He has consistently, when given a chance to work with the character, portrayed the Hulk as the personification of a very dark inner space of Banner.  In the "Ultimates", Hulk was portrayed pretty much as an uncontrollable side of Banner that dealt in heavy themes of sexual aggression and also---like "Old Man Logan", cannibalism.

 To not write a novel here---it's not necessary to get into all of the specifics.  It's just to say that Hulk is portrayed as a villain in "Old Man Logan" primarily, as is supported by the evidence, because that is Millar's personal take on the character as the writer from whose imagination the story came from.  Why Millar likes to depict Hulk like this can take up another conversation---but he does---and this is the reason why Hulk is depicted as he is in Millar's "Logan" story.  It isn't about how the story fits or what the "canonical" reasoning would be for it, or even if it is just accepted as alternate to canon, or otherwise.  This is like arguing about wrestling like it's real. It is entertainment, borne out of a creator/performer/writer, etc.  There are simply themes that Millar returns to when visiting certain Marvel characters, and with the Hulk---be tends to portray him as dangerous and indulging in darker parts of his personality as the Hulk that are uncomfortable, disturbing and not at all "heroic".