This is a good answer, thanks, and along the lines of what I was starting to imagine it was. The expression of a function of memory that keeps us averse from things which have given us a repeated, negative emotional reaction. We 'love' things that promote our well-being, and 'hate' things that detract from our well-being. Psychologically this would be advantageous because we are drawn to positive stimuli, and drawn away from negative stimuli, with obvious repercussions in a number of ways.
Hatred then becomes nothing more than avoidance behavior observed in a population. That seems a bit simplistic but maybe that is as good a definition as any. Prides of lions would be expressing hatred when they sought to destroy competing prides. Or would hatred be the act of expressing hatred? Or are we simply giving competition for resources and survival a new name?
For the record, and from what I've read, a behavior is something that can change in an individual. A trait is not something that can change. But then there are behavioral traits, meaning some behaviors are so hard wired that they are not malleable.
I think hatred would be the outcome of negative conditioning, which could manifest itself in a variety of behaviors. The word itself - hatred - would be the symbolization of the physiological phenomena. He 'hates' [thing] or he 'has been conditioned to dislike' [thing].
I'd think it goes deeper than competition for resources, and is likely one of the most basic functions of memory. Like a sensor drawing us to what we've been conditioned to perceive as good. We will seek out that which gives us a positive response - attractive members of the opposite sex, food that tastes good, safe spaces to inhabit, sane workplaces, and on and on. And we will avoid or destroy that which gives us a negative response.