Birth by Sleep My first theory.

I believe, that you have watched BBS leaked trailer with subs.
You remember last part of the trailer, when Terra says he no longer dont want to be a master, but he will use his powers for his friends.
So my theory is, that Maleficent (If i remember right, Terra and Maleficent worked for same reason at some point during BBS) promised Terra, that she will turn Terra into Master of the keyblade, if Terra helps him in search of seven princesses of hearts (Maybe). So Terra ends up doing evil things.

I have also another theory about Ven and his cutscene with Master Xehanort on Olympus colliseum.
When MX says, that Ven is starting to lose it (I dont what). Perhaps Master Xehanort wants Ven to be his apprentice, because Terra isnt intrested being his apprentice and Aqua is probably completely good person.

And in the end of leaked trailer, Ven wants that Aqua (Probably) will erase him, because he regrets his actions when he was apprentice of Master Xehanort.

 

The only problem is that Ven fought MXA(Master Xehanort Apprentice)so it couldn't be him.