Quote Originally Posted by kup
Bonecrusher was killed during battle - in the heat of combat. Demolishor was executed when he was completely out of the battle and defenseless.

That's a big difference.
You could also see it from the POV that Prime was putting Demolishor out of his misery - that after the fight had ended, Demolishor was fatally wounded and Prime was giving him a quicker and relatively more merciful death.

In the comic adaptation (which I suspect was based off an earlier revision of the script) Optimus Prime says, "I wish there was another way." as he shoots Demolishor - showing that while he is a realist, he still regrets having to do what he must (which distinguishes him from Dai Atlas who never showed remorse in ruthlessly slaying Decepticons). It's also in-line with the way he was portrayed in the first movie too, because after Megatron died he did say to him, "You left me no choice, brother." with regret in his voice.

Quote Originally Posted by TheRageOnTheRoads
And in essence when you get down to it the Decepticon's cause was no less geared to the protection of their species than the Autobot's. The Fallen/Megatron simply saw the world in and overly simplistic kill or be killed way (ironically exactly the mentality the autobots adapt in ROTF). There is nothing bad or irrational about that. You see that kind of thing play out in nature the whole time. They simply sought to eradicate all other life that could pose a threat to their species and ensure their own survival even if it meant blowing up a sun, killing others of your own kind. The ends justified the means.
Your last sentence sums up what differs between Autobots and Decepticons. The Fallen fell to evil when he broke the Primes' sacred rule about never harvesting a star that supported life, and later The Fallen corrupted Megatron, who was once lord high protector of Cybertron. (RE: Defiance - when's the TPB coming out?!)