Where is the incentive for them to consider the public interest, save for insofar as it is the same as the company interest?
It sounds like you think there is a problem: that executives being ruthless is not necessarily beneficial for society as a whole. But I don't think that's the root problem. Even if you got rid of all of the ruthless executives and replaced them with competitive-yet-conscientious executives, the pressures that creates and nurtures ruthless executives would still be in place. There are ruthless executives because the environment favors them in many circumstances.
I'm not arguing that the core of the problem is that business executives are too ruthless. But I do suspect that to the extent that the current system rewards ruthlessness, it may be purely or almost purely due to ways in which it deviates from a system that offers no perverse incentives from a societal perspective.
We have a tradition of treating ruthlessness in businesspeople as something of a virtue. Certainly, ruthlessness can help one get ahead in the business world, and companies often benefit from executives who're willing to put aside scruples while devising means of turning a profit. So ruthlessness in business executives can certainly be useful for businesses.
From a societal perspective though, businesses are only valuable to the extent that they increase the wealth and quality of life of society as a whole. Businesses are allowed (indeed, required, in the case of publicly traded companies) to attempt to maximize profits, on the presumption that in doing so, they'll enrich the broader society in which they operate. But there are plenty of ways in which businesses can increase their own profits without becoming more wealth productive, such as cooperating with competitors or establishing monopolies in order to keep prices artificially elevated, use of advertising to promote a product or service relative to equal or superior competitors, lobbying with politicians to slant the legal playing field in their own favor, and so forth.
I have reasons to expect myself to be somewhat biased on this issue, so I'm not sure how telling it is that I personally come up short of any examples of ruthlessness in business executives being useful from a societal perspective, when compared to business executives who're highly competitive, but compassionate, with restrictive senses of fair play. So does anyone else have examples of ruthlessness in businesspeople as a social virtue?