Whereas most Decepticons appear to dislike humans or disregard them, Megatron really does hate humans. I mean, why even bother? Unless... The whole reason he started the war in the first place and wanted to take control of the All Spark was so he could raise an army to exterminate any form of organic life. Perhaps Optimus Prime associated with smaller lifeforms in the universe and offered them protection, something his brother hated. I think that would make a heck of a lot of sense in a sequel.