Ironhide and Sideswipe would probably kill the 'Cons without a second thought, but this is Optimus freaking Prime we're talking about. Optimus Prime is like Superman - they're good because they feel they have to be; they both grant mercy to their opponents, with or without being asked. The only tiem I recall Prime not giving his opponent mercy was Megatron in the G1 movie, and that was just Megatron up to his usual treachery after their own battle. In DOTM, Megatron had just helped Optimus defeat Sentinel, who also had begged for mercy and was a fellow Autobot. What Bay has done would be like taking Superman and making him go against his personal vow of not taking a life, much like in Superman: at Earth's End.