Peter the Younger wrote:
I suppose we are to believe that, given the time to talk it out, the entity would have transmitted the equivalent of "Oh! I had no idea, I'm so sorry!"
That very well could have happened. There is no way to know without trying. Marr didn't even make an attempt. They could have destroyed it at any time. There would have been no loss in giving it a chance to understand and repent.
And it really does behoove them to extend this courtesy to those who wrong them, because they often make similar mistakes due to ignorance. Lets not forget that time when Federation miners accidentally systematically murdered thousands of helpless children. It can't be the only such incident. If the Horta could forgive them, and work with them peacefully, then the Federation must hold themselves to no lesser standard when dealing with people who harm them out of ignorance rather than malice.