The reality is that hatred between these groups of people--wealthy absentee land-owners, displaced farmers, business owners eking out a living, business owners cashing in on others' misfortunes, bosses, migrant workers, strike-breakers, and more--already existed. This novel depicted the realities Steinbeck observed. His point of view was clear, and his condemnation of the powerful elite who were taking advantage of the common man was scathing.
The political and social unrest of the day could not have been helped by such incendiary images, it's true. Tempers no doubt flared, and the anger was probably palpable. However, I still come back to the idea that the conditions Steinbeck wrote about were real. If that's the case, it probably acted as much as an agent of hate as it did an agent of change. Read it, get angry, then do something to make it better, said Steinbeck.