Point taken. Let me rephrase:
Why did Trump have to respond as he did?
The context is ambiguous, but the worst case scenario is that it was to attack Jeb Bush who boasted that GW was "keeping us safe." Technically he was, but only after 9/11. Obviously, for 9 months nobody was addressing the warning signs of an attack or problems within our intelligence community, regardless if you blame Clinton for creating that situation that Bush inherited and had for almost a year.
The beauty of the remarks is that it appeals to the Libertarian types and moderates/liberals who will interpret Trump as a "different kind of Republican" in his willingness to attack Bush where others will not utter a word. Hence Trump's high cross-over numbers from groups that are traditionally liberal.