There's another startup idea.
Is that the idea? Seems doable if you're not too risk averse, have no family and live in a country with weak extradition laws. Kidding, there's nothing illegal about any of this stuff or FB, Google and lots of other companies would not be in business.
FB would have a civil claim against you -- they paid several billion dollars for the legal right to all that user data!
Creating an app with the sole purpose of backdooring WhatsApp on a user's phone seems like it'd open you up to a lot of lawsuits. Ethically its a mite more questionable, but the original article is still unethical in that you're monitoring people without consent.
Like I said above, I'd do this just so that they'd crack down on it. It's still a "means justify the ends" argument, however, so you have to be quite comfortable with moral relativism.