What do the recurring thunder and crushed butterfly symbolize in "A Sound of Thunder" by Ray Bradbury?

In "A Sound of Thunder," which is set in the future, a man named Eckels pays $10,000 to go on a safari that takes him and others back in time to hunt a T-Rex. Anyone who goes on such a safari can only kill animals that would have died in a few minutes anyway and must stay on a special path that hovers six inches off the ground. Not the slightest thing from the past can be disturbed or the consequences for the future might be immense.

Unfortunately, Eckels gets frightened and runs off the path. When he gets back to his usual time in the future, he notices that everything is subtly altered and then finds a crushed butterfly from prehistoric times on the bottom of his shoe. Killing this butterfly symbolizes tampering with the past and, in so doing, changing the future. It symbolizes the fragility of the past and the importance of respecting that fragility.

The word thunder appears five times in the story. The first four times it is associated with the thunder of the T-Rex, both the noise he makes and the sound of killing him. It is associated with death. The final time in the story the word is used, it indicates Eckels being shot and killed.