It's time Sam finally got the answers he's long sought... some of them anyway.
Even today, I see fierce debates over exactly how much responsibility Terra has for tarnishing his own reputation with the rest of the cast. Among the Kingdom Hearts community, you'll see many people on both the "Terra Is An Idiot" front (my side) and the "Terra Did Nothing Wrong" front (not my side).
In a way, part of his problem comes from sheer circumstance. Any given RPG hero usually just listens to the first few people he/she encounters when they arrive at a new location. Almost every single time, these people happen to be trustworthy enough to for/with. Terra doesn't have that same luck. The "Terra Did Nothing Wrong" crowd would argue that he has no way of knowing that the people he first meets are all morally dubious, and thus can't be held responsible for being duped by them.
While I'll concede that Terra has very poor luck, I would argue against anyone who claims he is absolved of responsibility. It's only natural that people ask questions before they accept jobs or when they meet others. Most of us don't realize it, but whenever we meet someone for the first time, we often start by having them tell us about themselves and vice-versa. The fact that Terra doesn't do this is strange, especially since he's on a mission to learn about the goings-on in other worlds. Ventus has this problem too, but he's lucky enough to meet decent people.
To his credit, once the Evil Queen in Snow White asks him to kill the titular Snow White, he does start to ask questions: But that's only once he's asked to murder another human being. And even though he never had the intention of taking the job, he still accepted it and put himself in a position where it looks like he went for the kill.
A case can be made that he's not 100% responsible, but he does take some share of the blame. As a character trait, that even has the capacity to lead to some tense/dramatic moments. Unfortunately, Birth By Sleep commits one of, in my opinion, the greatest sins a storyteller can make when trying to build up drama. Unfortunately, it's too early to talk about it.