Megatron had been well informed of the possible future of his own aspirations and how terrible they'd grow to become left unchecked, so he is not entirely caught off-guard by Starscream's accusation and uneasy body language.

His own body was already stricken with his experiences here - silvery scarring from when he was attacked by Sentinel, and the still very raw gunshot wound that exposed some of his jaw mechanism from Spinister. A bit of a ghastly look, despite the tranquil expression on his face.

Of course, the lack of a badge and the unfamiliarity with the bot in question leaves him assuming this is some sort of neutral Cybertronian. So he raises his hands, placatingly,"I'm afraid I don't know much more than you do about how this came to be. That's a mystery that's still being resolved."