Lets to sort through the various motivations why the Allspark is sought. Megatron: He wants to build an army with the Allspark. Optimus and Sentinel: They wanted the Allspark to restore Cybertron. The Fallen: Primarily use the Allspark to find the Matrix in order to kill humanity out of petty revenge. Oh build an army to spread destruction in the universe. Megatron and Sentinel Prime: The compromise and truce meeting on Earth where the Allspark is. Megatron gets to build his army. Cybertron gets restored satisfying Sentinel. Optimus: He would rather have the Allspark destroyed rather than the Decepticons have it to conquer or destroy other sentient beings like Humans.