okay, I get why people say that Terra killed Eraqus, because he did injure Eraqus so badly that he may have been close to death. But, see the above photos. No matter what else he did,Terra did not finish Eraqus off. Eraqus’ expression in the top screenshot shows that it was a surprise attack that killed him, and Terra was already shown in a defenseless position and without his keyblade. So he’s not the one who did it. The second screenshot shows that Eraqus’ surprise attacker came from behind him, as Eraqus’ body fell in Terra’s direction (and not the other way around). Not to mention, Xehanort appear just moments later, from the direction the attack came from.
So, though I agree that Terra did inadvertently help Xehanort kill Eraqus, it was Xehanort who struck the final blow. It was also Xehanort who engineered the whole thing. If anything, Terra’s an Unwitting Pawn and not an out-right murderer.