Tommy was right -- this is emotional. Just seeing a never-ending war between undying spirits, fighting over their own power is something dreadful. But then you see the child -- the First Spinjitzu Master -- a being born from both worlds, born with both evil and good. Because of that, the Oni and the Dragons both wanted him, because he had both qualities. The fighting finally snapped him, and he left to live somewhere else where there was no war.
"He thought he could escape the war"...Part-Dragon and Part-Oni, the Master just wanted to live in peace, but his sons each had one predominant force. And so the brothers themselves were at war...only because the Oni and the Dragons dwelt in both of them.
Garmadon, the Oni-part of the First Spinjitzu Master, had the undying thirst for power and conquest. Wu, the Dragon-part of the First Spinjitzu Master, had the affection to settle his brother down, to keep at bay and to control the Oni. And what better way to defeat him "with the love of his own son".
Curse you, Hagemans. This truly is one of the most beautiful things I've ever seen.