I don't support the death penalty. I think that the act of killing someone who is defenceless and a danger to no one (as they must be when strapped down waiting to be electrocuted or injected) makes a coward of the state. However, some people really do make opposing the death penalty difficult.
I think that the documentary on Channel 4 looking at the 9/11 hijackers experience in the months leading up to the hijacking which aired a year or so made something of a mistake. There isn't necessarily a problem with attempting to humanise the attackers if it is done well and the documentary did make sure to highlight the betrayal of families that was involved. However, in choosing to end when they boarded the planes it managed to avoid dealing with the fact that the hijackings, as can be seen in the account of the recordings above, clearly required utter brutality.
While humanising the hijackers the film lost sight of the human consequences of their attacks. Holding a knife to a woman's throat, killing her, stabbing the unarmed passengers fighting for survival, barricading yourselves in the cockpit while bringing the plane down. These are the actions of cowards and monsters and humanising them cannot get around the fact that they possessed free will just like the rest of us and chose a truly evil path. This is a good illustration of the challenge that liberalism poses. Opposing the death of someone responsible for something so utterly horrible is an ugly business.