In as much as he failed to take swift and effective action agains the Iranian Islamonazies in '79, he does bear responsibility for the situation we now find ourselves in. Working for the terrorist Arafat and his Fatah killers didn't help matters any either.
I think it is more than unfair to say Carter is responsible for 9/11; I mean, that's beyond nasty. That's more like slander and defamation of character. Still, yes, he did the deeds that have given us the world that we have today. And it was unintentional. It has to do with one's worldview. If you don't understand how the world really works, then it is often difficult to solve the world's problems.