Why have Western capital execution methods gradually become more merciful?

There is a spot in the Piazza del Signoria that marks where Girolamo Savonarola was executed by Pope Alexander VI.
He was dangled by his neck over a fire so that he simultaneously strangled and burned to death. They broke his body into pieces and mixed them into the burning wood until there was nothing left.

That seems about as rough as it gets. It was in 1498.

Gradually, over time, Western countries have modified methods of authority sponsored killing, in an attempt to make them more quick and merciful, and then in most cases (except the US in particular) they stopped doing it altogether.
It’s good enough for me-I am opposed to the state or church killing a citizen or a heretic.
What is the reason for this “evolution” of attitude