Megatron dead? This one is in for a surprise. He's heard there are several Megatrons here.

"Oh, he died did he? Just keeled over one day, hm?" Rampage says, voice full of mockery. "Surely an Autobot didn't kill him!"

He's only guessing now, but it seems the likeliest option. If Sideswipe's Megatron had been killed by another Decepticon it hardly seems likely the war would have ended.

And a Megatron dying of natural causes just seems bizarre.