It wasn't about sending message. The theory is that Kelly could have revealed he was told to fake evidence, and could have provided confirmation of who knew what, and importantly, when.

It was about timing too - it was a critical point for Blair and Bush getting the war they so desired. Keeping in mind there was already huge opposition to the war, proper 1st hand evidence being revealed at that point could well have resulted in Blair having to stand down, and potentially even the British not joining the war. Which of course the US services would not have liked.