"Gibbs, Give me a reason not to kill Tony," said Ziva
"It would mean more paperwork," Gibbs replied.
Of course that wasn't really what Ziva meant. She would never kill Tony. He was like a brother to her. A very annoying brother sometimes, but a brother none the less. If he wasn't she would probably have killed him already, since she couldn't come up with a rational reason not to on her own. The reason Gibbs was chill about it was because he knew how Ziva felt about Tony. He knew she'd never kill him, but had a hard time admitting that she'd let someone into her heart. So he gave her a logical reason to latch onto. The Paperwork excuse.