To what killing was Tibbets referring when he said,
"We were determined to stop the killing"? What is ironic about his statement?

When he said that, he was referring to the victims of the world war all across Europe that initiated by nazi Germany. Many soldeirs and innocent civilians'lives were taken because of the military invasions conducted by the Axis Power.

That statement was ironic because Paul Tibbets was the pilot who dropped the Atomic Bomb to Hiroshima in 1945. Killing around 146,000 civilians with a single bomb.

i confess that there are several parts of this constitution which i do not at present approve, but i am not sure i shall never approve them: for having lived long, i have experienced many instances of being obliged by better information or fuller consideration, to change opinions even on important subjects, which i once thought right, but found to be otherwise. it is therefore that the older i grow, the more apt i am to doubt my own judgment, and to pay more respect to the judgment of others.