Gundam Seed

Cagalli Yula Athha:
If you were such a good friend, then why the hell would you kill him?

Athrun Zala:
I don't know. I have no idea why either, okay?

Cagalli Yula Athha:
Huh?

Athrun Zala:
We separated as friends and the next time I saw him we were enemies!

Cagalli Yula Athha:
Enemies?

Athrun Zala:
I asked him to come with us, over and over! Because he was a Coordinator, can't you see? He was one of us, that's where he belonged! It wasn't right for him to be on the side of the Earth forces!

Cagalli Yula Athha:
So you...

Athrun Zala:
But whenever we talked, he wouldn't listen! He fought against us, even hurting friends of mine! And then he killed Nicol!

Cagalli Yula Athha:
And so... that's why you did this to... your friend?

Athrun Zala:
He was an enemy! He wasn't on my side anymore! I had no choice but to kill him!

Cagalli Yula Athha:
You idiot!

Athrun Zala:
He killed Nicol right in front of my eyes! Nicol was fifteen. He loved the piano. He only fought to protect the PLANTs and Kira killed him.

Cagalli Yula Athha:
Kira only fought to protect the things he wished to protect as well. That still doesn't explain why he had to pay with his life! And in the hands of his best friend, of all people!