@anguirus23:
> With regards to the Heisei Gameras [...], he defends humanity but in
> Gamera 2 his psychic link to them is broken. So in Gamera 3, he still
> seeks to destroy humanity's foe the Gyaos but is careless and incredibly
> destructive as he does so.
@Battrarules:
> Gamera cares about getting rid of the Gyaos. Now, Gamera GOTU, he was
> connected. Gamera vs. Legion, he had the connection broken. Gamera 3, the
> man just annihilates Shibuya. Coinecedence, I think not.
Gamera was never connected to "humanity" and in Gamera 3 he does not act very differently. As we can see at the beginning of Gamera 3, there were also victims in Tokyo during his first battle with the Gyaos in Gamera 1. In Gamera 2, he completely annihilates Sendai while he was still connected to Asagi. In both cases there were probably less victims, but that was not because Gamera acted more considerately in those movies, but simply because the cities had already been evacuated when he appeared (there is a "The New York Times" headline in Gamera 2 stating "swift evacuation minimized casualties"). So, the Shibuya battle in Gamera 3 actually is not different from other incidents that happened before.
In fact, it's not Gamera who's changing, it's mankind. During the three movies, mankind slowly realizes that there is a war going on where human concerns do not matter much, a war between forces that do not regard mankind as the center of the universe. That is why the character of Mr. Saito is so important -- he represents the process of learning and the slow changing of mind. In Gamera 1, politicians and the military treated Gamera like an enemy, gravely misjudging the whole situation and underestimating the Gyaos. But at the very end of Gamera 3 (and after much damage caused by Gamera), the same people agree to fight alongside Gamera against the attacking Gyaos. They have finally learned that in order to survive mankind has to form at least a strategic alliance with Gamera. Fighting alongside Gamera does not mean that there'll be no more deaths caused by him or that Gamera will suddenly become a "friend" -- but if they don't do it, mankind will be doomed anyway.