I thought that the "Tell the truth" that Riddle snarls at Dumbledore was another example of his magic manifesting itself. Riddle used it as though he had found out that he could manipulate people into telling the truth. The book also says:

"He spoke the last three words with a ringing force that was almost shocking. It was a command, and it sounded as though he had given it many times before. His eyes had widened and he was glaring at Dumbledore, who made no response except to continue smiling pleasantly."

It seemed to me that it could've been Riddle's magic coming out, he is a powerful legilimens, and he could've discovered that he can make people respond to him by speaking a certain way. If he was just being imperious and bullying, I doubt that he would've said this and expected it to work on an adult, which he certainly does.

It's an interesting idea, though. :)