food for thought: Ven already knows about Eraqus’ death.
Braig accuses Terra of having killed Eraqus right in front of Ven
Ven’s naive, but he isn’t stupid. He may have noticed that rather than being shocked by or denying Braig’s claim, Aqua only becomes guarded and angry.
Ven himself doesn’t deny the claim, either. This is a significant change from all his mouthing off in response to Vanitas’ vague statements and Maleficent’s claims about Terra previously.
“But the Master said we can’t let that happen.” Said, not says. A minor detail, maybe, but considering Ven took Eraqus’ actions to heart enough to be very self-sacrificing at this point, then if he thought Eraqus was still alive, it seems more fitting for him to say “The Master says we can’t let that happen” to infer that it’s Eraqus’ belief, or “ruling” as their Master, and that it should be obeyed. of course, this would mean Ven knew before Braig said anything, but:
the last thing Ven saw was Eraqus and Terra both facing off in a rage. prior to that, Ven had no reason to ever think that Eraqus would ever try to hurt him – but he did. he never had any reason to think Terra would fight Eraqus – but he did. he never had any reason to think that Eraqus would threaten to kill Terra (“you will have to share Ventus’ fate”) – but he did. considering Ven spends an unknown length of time traversing the Lanes and then the Graveyard before meeting up with his friends, he may have thought through all this and realized that Eraqus – or Terra – dying was a real possibility.
(“then why didn’t Ven go back to stop them?” remember, he started to before Vanitas showed up and directed him to the Graveyard. besides also having Aqua’s safety to worry about now, Ven could have taken the mature route and realized that he was nothing but a burden on Terra right then, considering how easily Eraqus knocked him around; going back would mean being a distraction that would hinder Terra more than help him.)
also, Ven knows how Terra and Aqua are: severely protective of him. if Terra didn’t immediately speak up in the Graveyard to let Ven know that Eraqus was okay and put his mind at ease (and keep in mind that the archives say Terra was “battered and wounded” after his fight with Eraqus, so he probably looked pretty bad as a testament to things having not gone peacefully), this could well be taken as a bad sign considering Terra and Aqua now have a history of excluding Ven for his protection, regardless of what he wants.
the fact that Ven doesn’t even ask when he walks up to them could be taken as “oh he was naively assuming everything was okay,” but I think it could be the opposite: he didn’t ask because he didn’t want to know for sure. he didn’t want to make Terra own up to anything and add insult to emotional injury.
but this is all based on my assumption that Ven’s a lot smarter/matured over the game more than people typically give him credit for. and this is also assuming that Ven thought Terra was capable – emotionally – of killing Eraqus, which of course he wasn’t (BECAUSE HE DIDN’T), but again, Ven just saw a side of Eraqus that he never knew existed – arguably two, because Eraqus deceived and threatened him. at that point, and added to the fact that Ven had been feeling all around betrayed by everyone for a while now (and how, despite his criticism of Aqua, he DID start to doubt Terra even earlier than she did), he might not have known what to think anymore.