A few months later, a vast cloud of Ammonia and Methane seeps out of the ocean into the air, fish start dying and then land animals die as well as humans. They then realise that some of the viruses in the world were stopping Bacteria from becoming too numerous. Bacteria that makes Ammonia and Methane gas was being killed by viruses but now they weren't because the anti virus machines wiped them out. But it is too late, mankind is doomed.
What would be the alien's crime? They didn't make the anti virus machines themselves, they gave the instructions to people but they didn't tell them to make the machines. They may have known that some viruses are keeping us alive by killing harmful Bacteria. What if they said that it would be bad to build it jokingly so they still warned us but we thought they were joking. How would you even categorize their crime?