Because Khalid Sheikh Mohammed, 9/11 mastermind, was "tortured" via "waterboarding", lives were saved. Without the "waterboarding", lives would have been lost.
Before they were subjected to “enhanced techniques” of interrogation that included waterboarding, KSM and Zubaydah were not only uncooperative but also appeared contemptuous of the will of the American people to defend themselves.
“In particular, the CIA believes that it would have been unable to obtain critical information from numerous detainees, including KSM and Abu Zubaydah, without these enhanced techniques,” says the Justice Department memo. “Both KSM and Zubaydah had ‘expressed their belief that the general US population was ‘weak,’ lacked resilience, and would be unable to ‘do what was necessary’ to prevent the terrorists from succeeding in their goals.’ Indeed, before the CIA used enhanced techniques in its interrogation of KSM, KSM resisted giving any answers to questions about future attacks, simply noting, ‘Soon you will know.’”
After he was subjected to the “waterboard” technique, KSM became cooperative, providing intelligence that led to the capture of key al Qaeda allies and, eventually, the closing down of an East Asian terrorist cell that had been tasked with carrying out the 9/11-style attack on Los Angeles.
The May 30, 2005 Justice Department memo that details what happened in this regard was written by then-Principal Deputy Attorney General Steven G. Bradbury to John A. Rizzo, the senior deputy general counsel for the CIA.