One of the objectives of King Tranduil was to have all the races get along with each other. So that, they can work together to fight off against the disaster that will fall on them.
However, considering the scene in front of him, he knew that it was far from easy. Even the younger generation who should have least hatred was fighting as soon as they saw each other.
This was testament to how fragile the relationship between human and other races are.
It might have been easier for him if he didn't invite humans. However, considering how Elara was elaborating on how powerful Hiro and his friends will be in the future, and also the fact that humans have more geniuses than any other races, they couldn't just exclude humans who could become the greatest power in the battle with the calamity.
