A man who lost his father in the September 11 terrorist attacks is all in favor of capital punishment for alleged mastermind Khalid Sheikh Mohammed ... unless KSM wants to be a martyr.
Brett Eagleson -- the president of 9/11 Justice, an organization representing 9/11 survivors and relatives of victims -- tells TMZ ... he wants to see the Al Qaeda terrorists and KSM rot in prison if they are hoping to die for the cause -- but he wants them dead if they are hoping for a cushy life behind bars.
Remember ... KSM appeared headed for a life sentence as part of a plea deal with the U.S. government ... but the Department of Defense yanked the deal off the table at the eleventh hour.
The pre-trial agreement required KSM to plead guilty to conspiracy charges pertaining to the 2001 attacks ... and removed the possibility of a lengthy death penalty trial.
But, with U.S. Defense Secretary Lloyd Austin axing the deal, the death penalty is back on the table, as well as a trial ... and that's a win as far as Brett is concerned.
Brett's main point is that the absence of a trial prevents them from getting answers and holding the men and Saudi Arabia, accountable.
Still, Brett says the families of 9/11 victims were once again kept in the dark ... as was the case when the plea deal blindsided Brett and the folks he reps.
Brett's been demanding more transparency from the feds.
KSM and two of his alleged accomplices -- Walid Muhammad Salih Mubarak bin Attash and Mustafa Ahmed Adam al-Hawsawi -- appear headed for trial after all.
Brett's still a little skeptical though ... he's concerned the government doesn't want these guys to testify.