Is it Illegal?
That depends on the jurisdiction that applies. The legal implications of ignoring robots.txt may only apply under US federal law. If you are in another jurisdiction and broke a US federal law then the relevant authorities would have to start extradition proceedings against you. The general principal with extradition is that you must be accused of an offence which would be illegal in your jurisdiction so it all depends on your own circumstances.
Is it unethical? -
That's up to you and your individual interpretation of right and wrong. If you are acting in a professional capacity or on behalf of a corporation then you will need to adopt a different ethical code that applies to your position. Elements of this may however contradict your personal ethics.
The important thing to remember about law and ethics is that both are changing all the time. New laws are written and old ones discarded or updated. Personal ethics also change as societies views on issues evolve. It was once both legal and ethical to own slaves in the US. Now it is neither.