|"Mike and Bryan were really heavily influenced by film called Princess Mononoke for the ending of the show. Basically they borrowed from the movie, and then made their TV show. I can't make it back into a movie again, because then I'll be like 'stealing' from another movie, so I took the idea of what he [i.e. Aang] was going to do and make it more symbolic"|
— M. Night Shyamalan
Princess Mononoke (もののけ姫, Mononoke-hime) is a 1997 epic Japanese animated historical fantasy feature film written and directed by Hayao Miyazaki of Studio Ghibli. The design of Koizilla closely resemble the Forest Spirit from that film in its gigantic "Nightwalker" form. In the ending of Princess Mononoke, Lady Eboshi wanted to kill the Forest Spirit for the humans to triumph over the "Gods", which may be similar to what Zhao wanted to do with the Moon Spirit. Ashitaka (i.e. Iroh) pled to her for peaceful coexistence between them and humanity but she severed the head of the Spirit anyway, while it was turning into the Nightwalker form. The head (i.e. Moon Spirit) was captured while the body turned into a blob form (i.e. Ocean Spirit) killing everyone in its path while it searching for his head. Ashitaka risked his life to return the head back to the body and end its the brutal rampage. The original story elements in the Siege of the North included the tragic story of Princess Yue's sacrifice to save her people and Zhao's willingness to die as a hero (rather live as a failure) to the Fire Nation; those were kept in the movie.
Other similar story elements from Princess Mononoke were also not used in Shyamalan's film adaptation. The episode "Winter Solstice Part 1: The Spirit World" in particular, may been strongly influenced by the story relating to the forest destruction by humans, and its renewal with the planting of acorns: Hei Bai played the role similar to the Forest Spirit in its benevolent form but as a form of a panda, and its malevolent form as a large multi-armed monster instead of a ferocious "Blob". A key difference from Princess Mononoke was the inclusion of the Spirit World; in contrast from that episode, the Forest Spirit and the other supernatural creatures always reside in the "human world" rather than only during the Winter Solstice. The concept of the Spirit World that was first introduced in that episode, was included in the film adaptation by first mentioning it in Katara's narration of the Prologue, and later given a brief overview with Kanna's exposition to Katara and Sokka in the igloo.
As for the "symbolic gesture" in the movie, it should be noted that unlike in the episode The Siege of the North (Part 2), Aang was fully aware and in control of his immense Waterbending ability, i.e. his will was not overpowered by his past personalities and other entities with the Avatar State. Therefore, the movie finale is similar to the episode Sozin's Comet (Part 4: Avatar Aang), where Aang was in full control of the powers and abilities of a Fully Realized Avatar; he could finish off Ozai if he wanted to, but he chose not to.