I have mixed feelings about the darkness in Terra's heart, his confliction, and the way people react to him. During the mark of mastery when the darkness manifested, Terra was just as surprised as everyone else. This has always led me to believe that he most likely kept his fears, insecurities, and emotions in check. Even if he was anxious, I get the feeling that in general, Terra tends to be unshakable. He tries very hard to prove that he is still worthy of being a keyblade master and willing goes in search of the unversed and a chance to prove himself.
There is one thing in particular that has always bothered me. Aqua states that she's been to the same worlds as him and that she's seen what he's done. Now, Terra did not cause hate, discontent, or trouble in every world he visited. Off the top of my head there are a couple of really big things that he did that are concerning. One, he was manipulated into stealing Aurora's heart. It was also stated by Maleficient or Xehanort that Terra does not actually have the power to steal hearts. Although they certainly framed him for it. Second, the darkness does control him when he fights Braig. The desire to save Xehanort is so strong he does everything he can to save him; although he used the darkness his heart was only filled with the desire to protect and save someone he felt was important. This is something that Ven and Aqua are completely unaware of though. Lastly, he uses the darkness to defeat Eraqus so save Ven's life and it ultimately led to Eraqus's death. If Terra hadn't stepped in then Ventus would have died. Terra had a very difficult choice to make and he chose that Ven's death was wrong. I do not believe that he intended to kill Eraqus, only stop him. Aqua knows only that Terra fought Eraqus and is shocked to find out the reason. Xehanort used this to his advantage.
Those three things are probably the worst things that Terra did. His fatal flaw that makes him easy to control is that he is way too trusting. He believes what people tell him. He sides with the Evil Queen in Snow White's world so that he can find information and scares Snow White, but he has no real intention of hurting the girl. He protects stolen treasure for Hook and later goes to confront him when he finds out he was wrong. He fights in the coliseum to overcome the darkness and even assists Zack in overcoming it. He helps stop Captain Dark. Terra assists Stitch and becomes his first real friend. For me, the things Terra does well and for the light outweigh the bad. Terra is constantly trying to move forward and get rid of the darkness because he wants to be equal or level with his friends. He wants to honor them and he knows in his weakness with the darkness he might never be there.
Aqua cautions that his desire for revenge will only make the darkness worse and that it will only beget more darkness. Terra feels utterly responsible for his master's death. He needs to do something, anything to atone for that. He loved and respected Eraqus. He only stood up to him to protect Ventus. The situation in and of itself is lose-lose. At the end of the day, Terra only wants to protect his friends and loved ones but he isn't always sure how to do it. Sometimes, he makes mistakes. His heart is so strong though that after Xehanort takes over his body, Terra takes up his armor and fights as a Lingering Will. Even in his defeat, when Aqua willingly sacrifices herself to save him, he still looks after her. His keyblade is still there to look after her. Now, here's the thing about that. His heart is so strong, so geared to protecting others that even without a physical body he manifests his armor and uses his keyblade to save those he loves. The darkness may have been a part of him, but it was love, light, and friendship that made him.