Autumn Raine brilliantly uses pedal-operated recordings of himself playing every part to pull together Terra's theme from Birth by Sleep.
Terra kind of gets the shaft within the fandom. Yes, he's pretty gullible. Yes, he made a ton of bad decisions. That doesn't make his path into the darkness any less tragic.
There's no doubt that Terra reaches his lowest point while fighting his father figure, Master Eraqus, in the Land of Departure. While he fought to protect his friend Ventus, Terra was also being manipulated by Master Xehanort, who struck Eraqus down after Terra gave into the darkness and won the match.
Terra's theme builds in parallel to his rising feelings of guilt, envy, and anger that eventually overtake his heart.