He aw that the Time Lords became just as cold and unfeeling as the Daleks during the Time War. In the Time War, both the Daleks and Time Lords wiped out entire planets and civilizations, made them as if they never existed, as both sides constantly used time travel to rewrite history. The doctor could not take such widespread death and destruction, so when he had a chance to "bottle up" both sides in a far flung dimension or time stream, he took it. He feels great guilt for betraying his kind, and causing the seeming deaths of Daleks and time lords, but he did it to save the lesser civilizations caught up in the Time War.

He was aware that the Time Lords became just as cold and unfeeling as the Daleks during the Time War. In the Time War, both the Daleks and Time Lords wiped out entire planets and civilizations, made them as if they never existed, as both sides constantly used time travel to rewrite history. The doctor could not take such widespread death and destruction, so when he had a chance to "bottle up" both sides in a far flung dimension or time stream, he took it. He feels great guilt for betraying his kind, and causing the seeming deaths of Daleks and time lords, but he did it to save the lesser civilizations caught up in the Time War.