What does it matter if he thinks he's right? From an objective standpoint we can say it's wrong to kill people just for researching history. Just from looking at Ohara, there is no standpoint from which you could say Akainu or the elder stars were in the right.
There's no such thing as an objective standpoint in this matter. Look, this is way too high level for a manga thread, kind of off-topic and I know people are going to be doing strange looks, but to answer your point: what you're describing are the norms and cultural rules that we have decided in our society.
In the real world, people are in fact murdered for researching. Not for researching history -- as far as I know there are no ancient devastating weapons to search the history books for in our real world -- but for researching other things that can weaken the government.
In this scenario, in One Piece during Ohara, from the governments standpoint, they wanted to ensure that no one could ever remake an ancient weapon and potentially kill millions; the people of Marijore in particular no doubt. How you can say that their decision was "objectively wrong" is just ignorant. Seeing situations from other peoples position and thinking of their motives and their perspective is vital here; even if it's just a rather shallow fictional character, sure.
Again, this is way too high-level for GAF and we should really let this discussion go, but these are philosophical questions. It's important to keep in mind that people are shaped by many things; in particular the society in which they were born and the beliefs they were taught as children. And when you imply that we can all agree that it is "wrong", well, you must keep in mind that someone told you what is wrong to begin with, and that someone is your government and your society. There is never such a thing as objectively wrong in these matters, however what you're describing are part of the basic principles of norms and morale.
But yeah anyway, a lot of people wanted to call Akainu an asshole and that's alright. Personally I just don't see it that way.