“In the moment when I truly understand my enemy, understand him well enough to defeat him, then in that very moment I also love him. I think it's impossible to really understand somebody, what they want, what they believe, and not love them the way they love themselves. And then, in that very moment when I love them –"
"You beat them." For a moment she was not afraid of his understanding.
"No, you don't understand. I destroy them. I make it impossible for them to ever hurt me again. I grind them and grind them until they don't exist." (13.127-129)
Ender may be a product of war, and he may be very good at it, but in some ways, he’s also a casualty. Think about it. If Ender loves the enemy, then destroying the enemy is always going to be a little painful. (Or very painful.) This is part of why Ender seems like a sad character in this book: the thing that he’s so great at (war) is painful. (Although the fact that we see Ender’s pain but not the pain of, say, Stilson, does strike us as a little odd. It’s important to remember that Ender isn’t the only casualty of his wars.)