User blog comment:KillRoy231/My review of the Deadly Six/@comment-477304-20131021001503/@comment-26973802-20131021030253

Right, I guess Mephiles is still the darkest then. With Mephiles and the Deadly Six, they don't seem to have a clear motive for destroying the world other than just doing it for the hell of it. Black Doom wants to harvest the humans as an energy source. Eggman Nega, well, I guess he never gave a reason for wanting to destroy the world, so I guess you've got a good point there

In Sonic Generations, Metal Sonic - either of the past or reverting to his classic form for the occasion using his liquid metal shapeshifting powers - is destroyed by Classic Sonic. Classic Sonic kicks him, and he explodes. If it's present day Metal Sonic transformed, nuff said. If it's Metal Sonic of the past, we can assume that when the timeline is restored, present Metal Sonic disappears.