While Plato is obviously correct in his interpretation of good and evil in humans, his interpretation using a soul has been a disaster for humanity.

Humans have two primary instincts. The "red of tooth and claw" competition most common to nature and the cooperative ability we developed when we became ground dwellers (about 3 million years ago - Lucy -Australopithecines). That cooperative nature is shown in the fossil record by the large increase in brain size. ... About 70,000 years ago was an evolutionary event (shown by tools, art, burial customs, etc. ) where humans became much more efficient and needed cooperation less to survive. Cooperation moved back to competition. We started killing each other, which peaked with Rome. About then, religions and philosophies developed that moved us back towards our cooperative instincts, partly because everyone hated the war.

Again, you have instincts for cooperation and competition, the two horses. You can easily find both in yourself and others. We interpret them as good and evil. Knowing about those is important because humans will only have a future is we select the cooperative strategy.

The problem is that these are instincts, innate knowledge. Plato thought that innate knowledge came from the soul. I guess that's reasonable given that even today with our knowledge of biology, most people can't get their mind around instincts. The belief that innate knowledge comes from the soul certainly lasted until Freud and Jung. An incredible number of mistakes have been made based on Plato's description of the soul, as well as his pursuit of perfection.