What do you think Montag is expressing regret about when he calls himself "an awful fool, an idiot"?

Montag is expressing regret over the lost lives and feeling his own part in the deaths. He is second guessing his actions, debating whether to end it all and turn himself in, and finally deciding that it's all worth it.... that he needs to save what he can.

No, we'll save what we can, we'll do what there is left to do. If we have to burn, let's take a few more with us.