There is also the remote possibility they didn't know they were carrying bombs. Someone could have said, "Hey Achmed, how 'bout hauling this load of Korans to the mosque across town for me?"
I would not be at all surprised if there are frequent "training" missions, where people are assigned to carry dummy packs from place to place, ostensibly to measure travel times to certain locations or the vigilance of security personnel or the like. It would be a simple matter to replace the dummy packs with the real thing on one of these "training" missions and the bombers would think it was just another milk run.
But that still presupposes a willingness to be involved in mass murder, even if only peripherally. These idiots were either evil or stupid, but quite probably they were both.