No no no.
They killed the Blackrock Orcs because they attacked them and didn't let them speak with Archimonde, but your reason too is right. Plus, the Scourge was created to summon the Burning Legion, and was not ment to just go randomly attack anyone who is living. It would have no tactical, technical, and strategical use to attack Gilneas, plus, they had a huge badass wall, so how were they even supposed to get in?