We advertised a service that was good for the people who wanted to protect their privacy. We, like other sane people, do not support and strongly condemn immoral uses of mixers.
The authorities go after the big ones, not the little ones. Furthermore, the advertisers have not committed any illegality or done anything wrong.
Users who participated in the campaign do not have any responsibility for how the company's business is done.
TryNinja gave the example of Google, but the same happens with newspapers, television, radio. They advertise many products and services, which later become mean. They just did what they were paid, within the law, everything else is beyond them.
Therefore, this is a concern that no one here should have.