Capítulo 796: The Patience Of Immortality.

The major reason why he began to hate the realm lord is because of the theft of life essence from the high life plane by demons. The demons didn't destroy the life trees. They just took their flowers but that was enough to make him angry at the realm lord. Now he knows he was being naive and ignorant back then. His thoughts and opinions of the realm lord don't matter without the strength to effect the change he desires. And the changes he wanted back then we're stupid too.

Maybe he could have stopped the Invasion of his plane by demons if he had the strength to. But he would have faced a lot of opposition from the other powerful high elves even if he had the strength. It is not like there is no one in the entire plane or across the entire ancient history of high elves who didn't know what the realm lord was doing by allowing demons to attack them.