If we accept that Kirk fired on Nero because he honestly felt that he could escape through the wormhole then we also have to consider the probability that the same thing actually happened to Vulcan and Nero didn't actually kill everyone on the planet. Perhaps Vulcan was just shifted to an alternate universe and/or back in time. Reasonable doubt works both ways.
When Spock and Nero travelled back in time, they both entered a black hole that was created by red matter. When the Narenda was being destroyed at the end of the movie, the black hole was inside the ship. We saw it being bent, torn and crumpled. If the Narenda could reasonably be expected to survive passage through a black hole to the extent that Nero and his crew would survive then the same should apply to Vulcan. We also saw it being distorted. Perhaps a red matter black hole tears apart whatever falls into it but then reassembles it on the other side. Much like a transporter would reassemble you and not move you whole from one spot to the other.
If Kirk had reasonable doubt that Nero would die then Nero could use the same defense. Kirk had no proof that Nero killed everyone on the planet and then destroyed the planet. Never having encountered red matter black holes before the on;y thing that he could say for sure was that they transport objects back in time. For all we know everyone on Vulcan is fine, even the guy that got flattened by the statue. When you emerge out the other side, time runs backwards and you end up exactly in the same physical state you were when you first entered the black hole.