Art never had a definition as specific as you're making out it did.
You're absolutely right.
Art has always been defined by it's aesthetic, which is in turn defined by the people of the age. As time changes, the idea of what art is changes as well. Therefore, it has a fluid definition.
Let me give an example, the Rite of Spring. Igor Stravinsky very experimental with this particular ballet. Unlike most ballets, it had no even rhythm. It's rhythm is constantly interrupted is full of random bits of disjunction, hopping randomly here and there and back again in a pulsing, pounding way. Not anything close to what people would consider ballet. When it premiered in Paris, the ballet goers were furious that it existed. They saw it as a mockery of their ballet culture. One newspaper described the performance as "a laborious and puerile barbarity". What was barbaric was the crowd, which eventually grew so loud that the orchestra couldn't be heard, and the dancers could only barely hear their dance instructor in the wings who was desperately trying to give them the proper tempo. The audience eventually started to bombard the orchestra anything near them that they could throw. They were able to finish the Rite of Spring that night, and nights afterwards with reasonable success. The ending was actually received well enough. Very few in their right mind however would have called it art, save for those who were involved in it's production and were enamored of the chaos.
Now? Hell yes it's art. It's flipped the standard convention of music and turned it on it's head. Melody? Didn't need it all that much. Rhythm? Let's make it chaotic. That way it sounds like complete madness is taking place on stage. Igor Stravinsky pulled it off and the end effect was brilliant.
In the case of Hatred... I'm with the Parisians. You can call it art but for now I want to willfully distance away from it. I suppose if I had to give any sort of logic to that reasoning, it's just not the culture that I want to adopt into my aesthetic.