Did the SEALS summarily execute Bin Laden when they could have captured him? Maybe. The official story now is that bin Laden was not armed, nor was he using his wife as a human sheild (she was charging the lead SEAL and got shot in the leg), but he was standing in the middle of the room and was shot because the team saw him reach for what they thought was a weapon.
Though, Obama did indicate in his speech that we first captured Bin Laden, then killed him, which I thought was interesting.
I'm willing to believe the SEALs when they said they shot him because he was going for a weapon.