@kevo_koma:
Hmm you are right but you also have to consider that maybe he thought that somehow the archeologist would pass on the information to someone else and set off a domino effect at least that's how I think he reasoned, why deal with chances when you can just blow the problem up.
This does raise an interesting question as to what truly happened in the void century, if we consider that someone as seemingly honorable as Sengoku knows about it and is willing to protect its secret then maybe it isn't as black and white as we might think and considering that there are hints that it will truly bring about a world engulfing war maybe its better if it was secret.
you know, people on ships could be informed about archaeologist to be extra cautious. On the ship packed with people it's rather impossible to transfer information without nobody knowing, so investigation would be relatively easy, especially since the archaeologist was a child, therefor unlikely to come up with complicate deception. I worst cases scenario they could escort just the one ship she was on and kill everyone just on that ship, without killing the rest - it's still bad, but not as bad as killing all those unconcerned. Furthermore, Robin didn't know much about Ancient kingdom, she only knew how to read phonegiphs, it isn't the kind of skill that can be quickly learned.
No matter what Akainu tried to protect, no matter if he was right or wrong about need to protect it, what Akainu did in Ohara is evil and inexcusable. Worse, it wan't one time occurrence it's his way of taking on his duty.
Sengoku may have been willing to cover for Akainu's actions for some reason, maybe because the need to protect the secret, but he decided that Aokiji would be better Fleet Admiral than Akainu, so he is probably aware that is bad new for the Marines and citizens, taking into account Akainu is shown as much more dutiful upstanding than lazy Aokiji.