[Aquinasfan]

"...when this is the only practicable way to defend the lives of human beings effectively against the aggressor."

The technology that makes it possible in some societies to imprison murderers for life, without risk to society, was not available or even imaginable at the time of the Council of Trent, so it's hard to imagine the bishops saying, "but if it becomes technically possible someday to imprison murderers for life... etc."

This is a refinement of doctrine, not a change in doctrine. The Church is not repudiating the right of the State to execute murderers.

[murphE]

This is a refinement of doctrine.

It's not doctrine at all. It's editorial opinion that many well meaning, pious Catholics have been misled into believing is doctrine.

[steve-b]

The technology that makes it possible in some societies to imprison murderers for life, without risk to society, was not available or even imaginable at the time of the Council of Trent

Nobody in the sixteenth century knew how to build a cage?

You learn something new every day....