Makes perfect sense to me. As is noted in The Evil Genius Guide to World Domination
"The dead are easier to control, but far less fun to do so."
And in How to Conquer the Galaxy for Dummies
"People fighting to survive generally fight harder than people fighting for lines on a map. Try to convince them you intend on being a neighbor rather than utterly destroying all who oppose you(no matter how fun that can be). An enemy demoralized can be more pliable than an enemy near death"
Of course there are different thoughts on the matter, but seems that unless you want to watch your back every second of every day for an attack from within and without your empire, you have to make some semblance of not wanting to kill everyone. Otherwise it tends to make people who are not being conquered more willing to help those that are.
*wondering how many people are searching for those books now*