Optimus Prime says: " As if bent on one last, selfless act of sacrifice, it took our sin... and swallowed it whole."
According to the reviews in TFW2005.com:
During the apocalyptic conflict, however, it was said that the creature committed “one last act of self-sacrifice, it took our sin… and swallowed
It seems to me that Thunderwing did something great (self-sarcrifice) to save the planet. I am guessing maybe at that time, Thunderwing realized that he did something wrong , and then he gave up his power and let the fire from Autobots and Decepticons destroy himself. That is my understanding of self-sarcrifice.
However, this detail never got further explained in the following issues. I sitll believe Thunderwing is a good guy in nature , but twisted by his technology.