In regard to the political organization of a society, your question has been extensively and fruitfully debated for centuries, for instance in Plato's Republic, in the Federalist Papers, and in Rawls's work. The discussion shows that social rules, practices, and institutions exert great influence on character and conduct. Ethical conduct is more likely when ethical standards are clearly formulated and discussed, when there is transparency and accountability in decision making, when counter-moral incentives are restrained or suppressed, and when ethical considerations are routinely integrated into decision making processes. In regard to the organization of a business, similar desiderata apply. So, yes, structural design is important, and you do well to pay as much attention to it as you do. Still, this does not show that prevailing attitudes about morality are wrong to put individual agents at the heart of the matter. You are such an agent, and, without your thoughtfulness, your business will not be well designed. Moreover, a culture of selfishness and corruption may render meaningless even the best-crafted rules or governing documents. Such documents are one crucial component of a corporate culture which must be sustained also by people working together at the firm who must, for the most part at least, understand and share your commitment to operating the firm in an ethical way.