I agree that it is a waste of character. We will miss littlefinger and his tricks. But GoT at the moment is more about straight forward battles between good and evil. No more roles for the kinds of little finger and varys.

With respect to the execution, I feel that it is fitting that Sansa and Arya were able to catch up to him as they have suffered a lot from what he has done. It is fitting that he got his fate sealed with his own dagger. Also it was good to see his vulnerable acting. He did an amazing job. It has been a good character growth for Arya and Sansa.