Well the problem in keeping him alive was that his accomplices would have tried to get him out by hijacking a plane or bombing some place like that. One of the key mind behind 9/11 was captured in India and Air India flight was hijacked( from Kathmandu, Nepal) and was flown to Kandahar, Afghanistan. Indian government had to release him and 2 more to save civilian lives. Yet if he was to be killed he was to be entitled a fair trial, then that would had been a civilized act. Because taliban and Al-Qaida did this kind of executions, I'm just searching for the difference here.