Why do employees sometimes make excuses for behaving unethically?

The quick answer to this has two parts: first, employees tend to not want to think of themselves as unethical, because that places them into the same category as people like Heinrich Himmler and Charles Manson, whose gross violations against society have echoed decades after they stopped acting.

Second, employees want to avoid having to answer for such actions, especially where they believe that the actions were well-justified.

In general, current thinking for resolving the problem of unethical behaviors is actually training in the area of ethics and morals (from a philosophical perspective, rather than a religious or societal one), after which any breach of ethics leads to dismissal, to preserve the company's interests.

A few days of training is well worth the investment, as it dramatically reduces legal risks and increases employee productivity by giving employees a framework in which decision-making is faster and more productive.