The problem with eye-for-an-eye punishment is that it doesn't take into account relativity. Deprive someone of water for a few days in the United States, and this is quite torturous indeed - but deprive a child in Ethiopia of water for two weeks, and this would just be normal life.
Knowing that you are going to die peacefully is comforting, especially when you know when. You have time to come to terms with it. The death penalty verdict for the monster that killed those kids is perhaps the most wonderful thing that could have happened to him. His life had no potential, he was not going to do great things, he would not raise a loving family or contribute to society. His life was worthless, so to take it from him on any level - well, I'm sorry, it's just not enough.
Accepting that life is unfair and that there is no way to truly find justice in some circumstances is upsetting, but necessary.