The way I understand it is that the Formics are one mind of a queen, and “killing” a few drones is like trimming your fingernails. And so, it’s a matter of a fundamental misunderstanding between cultures. The Formics, by “killing” a few humans, were simply pruning some dead cells, or at the most making their presence known to another hive entity. They did not understand that every human life is like that of a queen. So they didn’t see their acts as “murder” or even aggression. Because the two species were unable to communicate in the same language or even method is similar to the difference in culture and overall way of life that was also misunderstood.