I always like these 'outsider' looks at the team. :-)

I figured Ari picked Kate because he knew her the best---killing her was less about Gibbs and more about Ari in some ways, at least in my head. Ari knew targeting the people Gibbs thought of as more vulnerable would get to him, and he'd try to protect them---leaving everyone else that much more vulnerable. Was a classic misdirection. He got to Gerald because Gibbs was focused on keeping Abby safe. He could have just as easily gotten Tony given that Gibbs had him trailing Ziva.