"It is a far, far better thing that I do than I have ever done; it is a far, far better rest that I go to than I have ever known." Sometimes the story ends with the character on the way to the final sacrifice rather than after they die. That doesn't mean it's a cliffhanger; it just means the writer is exercising discretion, or letting the character go out with dignity.
I agree, but had
there been another installment, we'd have probably found out whether Keller permitted Bonifay's sentence to be carried out. There was a lot of debate by fans about whether Keller would weaken or not, and try to save him.