About the new Transformers movie?

Okay what’s the deal with the glasses? So Megatron is in the Arctic and he frozen and he etches the glasses with the code that says where the All Spark is which apparently is in Hoover Dam. So if he knows where the All Spark is what was he doing in the Arctic? If he had scanned the Earth from space and knew where the All Spark was why didn’t he go there first? I’m guessing that they built the Hoover Dam around the All Spark because they couldn’t move it. Also why is it that the power generated by the All Spark only creates bad Transformers not good ones? So why would they want to get the All Spark back and rebuild Cybertron with it since all it is doing is making bad Transformers?

I did enjoy this movie very much and normally I don’t worry about trivial little junk in the plot that doesn’t make sense but I was just wondering if anyone knew the answers to these questions or could hazard a guess that makes sense.