@The:
Maybe Corazon is a fan of absolutes like Akainu.
Despite him being serious, I still wouldn't take Corazon for word. We saw him a few times get very emotional over small things (main one crying over Laws story). He was also quite driven to help Law on his emotion of just wanting to help out. While he might be straight faced, he seems very keen on acting on emotions. Also, seeing how Doflamingo shot his own father, I can see Corazon at that point seeing Doflamingo as absolutely evil. Doflamingo is like Corazon on acting on emotions as well, but instead of good intentions, it's bad intentions and the negative emotions he focuses on.
The issue is not what Corazon is telling us, it is what Oda is telling us. Like, in this scene, is Oda telling us that Akainu is making a reasonable and valid exclamation? Is there any doubt what we, as readers are supposed to feel about him? Readers personal feelings on Akainus policies may vary, but Oda is very clearly telling us "wow, look at this guy! This guy is going overboard!"
Now, the entire Law-flashback scene is set up to very clearly tell the readers what they are supposed to feel about Corazons statement. We are to accept it as fact; it is not supposed to be ambiguous, Corazon is not presented as biased. Lets break down the scene:
First, Corazon says that he cannot believe Doflamingo is the son of their parents, that he's not human. This is immediately followed by a flashback to the Donquixotes past. The fact that Corazons words trigger the flashback shows us that the events will relate to his words.
In the flashback, Homing and Rocinante are scared and begging for death- but Doflamingo is angry and vengeful. This- importantly- is met with surprise from his family. We then get a large panel (suggesting this is important) of Doflamingo threatening murder, which is met by another "!!?" and shocked expressions from the mob, showing that his words are unexpected.
Cut back to Corazon who says first thing that Doflamingo was "Born Evil", which again, comes as a followup to the flashbacks events.
So the flow of events here, from a storytelling perspective, is: Corazon makes a point–> flashback illustrating his point-->Corazon reiterates his point. The purpose of the flashback-within-flashback, Odas point, is to demonstrate Corazons point, namely how Doflamingo is an evil inhuman monster.
Now, the problem is that we- the readers- feel that the awful events in Doflamingos childhood could drive someone over the edge and make them threaten murder in the given context, especially given their upbringing- for us, it doesn't fit that Doffy was "Born evil". But that is what the scene, what Oda tells us. Doffy would've been evil either way. Corazons statement is never challenged, and the topic is not even raised when the two brothers face off; theres no "Why are you fighting me? Remember what the world did to us! Don't you wnat to make them pay?" or "I know we had a shitty life Doffy, but thats no reason for this" between the two brothers.
Given the context Corazons statement is given in, combined witht the fact that it is never challenged and Corazons overall presentation as a guy to root for, It becomes not simply some biased opinion Corazon tells the readers, but something Oda tells the readers.