Despite the fact that Nomura purposely made Terra like Riku, at least Riku isn't as stupid as Terra. There was no evidence of Terra's trusting nature. He was never over confident. He was in denial the whole time. He kept telling himself he can overcome the darkness because he feared it. Even then, there is no excuse for basically using the Princesses to further his own desires. He only went around to find Xehanort and did not give two shits of the girls. I still stress the point where the Queen straight up asks him to kill Snow White and he doesn't see anything wrong with it.
It also helps not to sound rude when you just entered a discussion.
He thought that Snow White might be able to help him find XH so he decided to go find her and not cause trouble but I don't think he actually ever intended to kill her. He didn't say no to the Queen because why start useless drama when you could just go find Snow White and possibly get clues. After Snow White ran into the woods, he had no choice but to go back to the Queen in hopes of the Queen asking the mirror, despite the fact that he didn't kill Snow White.