In Descent Part I, Admiral Nechayev arrives on the Enterprise and says the following to Picard (emphasis added):
NECHAYEV: As I understand, it you found a single Borg at a crash site ...and eventually found a way to send it back to the Borg with a programme that would have destroyed the entire collective once and for all... Now I want to make it clear that if you have a similar opportunity in the future, an opportunity to destroy the Borg, you are under orders to take advantage of it.
According to Memory Alpha, genocide is defined as:
a deliberate, systematic destruction of an ethnic, racial, religious, or national group.
While the Borg are made up of thousands of species and therefore have no race, ethnicity or religion to speak of, it is easy to see that they are a nation unto themselves.
Given this, did Nechayev order that Picard should commit genocide?
Follow up: One of the tenets of the Prime Directive is not:
Taking actions to generally affect a society's overall development
If the Admiral DID order genocide, was it a legal order in light of the Prime Directive and considering that killing the entire species would obviously affect it's development?