Deep sorrow, yes.
Regret?..regret that it was necessary, yes.
Diminished all our humanity? Absolutely not.
I think he's interpreting Donne* (also a cleric) here.
Each man's death diminishes me,
For I am involved in mankind.
Therefore, send not to know
For whom the bell tolls,
It tolls for thee.
but I don't agree with the interpretation. To risk one's earthly life to put a stop to evil, as the bomber crews did, is one of the highest human virtues.
The necessity of fighting and killing, for any legitimate action by armed forces, diminishes one's happiness (which is another sacrifice made by fighting personnel), but not their humanity.
*
For whom the bell tolls a poem by John Donne