I guess I would argue that, for the purposes of ethics, corporations don't exist. They are just people-groups organized (more in a flocking sense than a regiment) around some particular goal.

So, to rephrase the question, can an arbitrary collection of human beings behave ethically? The answer largely depends on your worldview, of course, but all unethical actions are taken by humans. Sure, it is a useful mental model to think of a corporation as a gestalt whole for some things, but that's all it is - a mental model.