My theory (it may cointain some Mockingjay part 2 and book spoilers):
I think Plutarch is reading what he wrote for Coin and he’s smiling because she’s in his hands. I mean, okay, this may sound stupid but I think these: “When you fired your arrow at the force field you electrified the nation. There have been riots and uprisings and strikes in seven districts. We believe that if we keep this energy going we can unify the districts against the Capitol. But if we don't, if we let it dissipate, we could be waiting another 75 years for this opportunity.” are not the words of Coin. We know that Plutarch is a good manipulator and we know what he’s capable of. We also know that at the end of the movie he’s mouthing Coin’s words meaning he wrote that speech. He doesn’t care about Katniss or anyone else; the only thing he cares about is how they could win the war.
My point is: what if Plutarch is behind the bombing at the end and not Coin? What if he told Coin about a final Hunger Games with the children of Capitol people in it? What if he’s our “bad guy” and not Coin? Winning at all costs...