Complex sentence: Socrates points out the human tendency to corruption by power and thus the road to timocracy, oligarchy, democracy and tyranny: concluding that ruling should be left to philosophers, the most just and therefore least susceptible to corruption.
Split into simpler sentences: Socrates points out the human tendency to be corrupted by power leads down the road to timocracy, oligarchy, democracy and tyranny. From this, he concludes that ruling should be left to philosophers, who are the most just and therefore least susceptible to corruption.