Probably the simplest possible motivation. He wanted more power, and he didn't care how many people had to die for him to achieve it. In terms of your philosophies you could say he had all passion and no attachments.

[His voice is wry but it's still a very sore topic. His own fault for letting the conversation end up here, though. He brightens his tone again with sheer force of will:]

I'm sorry, Imperial politics can't be of much interest to you. It was a very small conflict compared to what you're used to.