Thanks for the question.
I think in your question you meant to say Pope Francis, in updating the Catechism, is changing Church teaching.
Pope John Paul II did not change teaching, (nor did he author Humanae Vitae, Pope Paul VI did) but rather he [Pope John Paul II] suggested it was unnecessary and sometimes problematic in our modern times.
Pope Francis, however, has contradicted the infallible, consistent teaching of the Church for millennia, and therefore he is wrong. He cannot change the teaching, and therefore whatever he said regarding Capital Punishment is irrelevant; it is merely his opinion. It is unfortunate that he has the ability to inject this into the Catechism, but eventually the Church will correct it at some point — that is a matter of time.
No one can change or contradict the consistent teaching of the Church on faith and morals, not even a pope.
Unfortunately, much of what Pope Francis says has been filled with confusion, controversy and inconsistency. This has led to many second guessing the validity of his papacy, and you will find no shortage of theories on the genesis and outcome of these scandals associated with his papacy.
Here we can only pray for the Church, but we know that the Gates of Hell won't prevail against Her in the end.
These are difficult times indeed.
P.S. You can find a good discussion of the issue here. While it is over an hour, you will find Dr. Taylor Marshall is a good resource to utilize.