Okay, this is really irking me. During the battle with Trypticon, Air-Raid, Silverbolt and Jetfire swoop down to deliver overshields. Immediately after delivering them, Air-Raid is smacked away and I clearly see him HIT A WALL AND EXPLODE. Silverbolt and Jetfire rush in and are blasted to bits with their smoldering remains smacking to a wall and exploding. Yet after this happens, they are seen next to the main autobots in the final cutscene completely unharmed. What gives?
I'll have to play the game again, but I don't remember any of them hitting the wall and exploding. Getting hit and careening offscreen, yes, but not hitting the wall and exploding.
Partly because Starscream is Cybertron's foremost expert on Dark Energon and partly because Starscream "intrigues" him.
One word; Attrition. The war has apparently been going on got several thousand years, without ceasefire. In just one battle, hundreds of combatants on both sides are destroyed, and those are just the battles you participate in; there are hundreds of others taking place all across the planet simultaneously. As they are technicly robots, I suppose you could just manufacture more, but as there are "young" Autobots like bumblebee, it would seem that their A.I. needs time to mature. Furthermore, with Cybertron being one big city/machine, there's no place to mine raw ore for manufacturing. How do the two factions keep up with the constant cost of war?
After Megatron defeats the previous Prime and takes control, he gives Optimus and the Autobots the option to leave Cybertron or perish. What do the Autobots do? Make plans to leave. Megatron still tries to kill them anyway!
Megatron is a reckless megalomaniac with self-destructive delusions of grander. Hypocrisy is perfectly in line with his personality all things considered. (He is an actual "Saturday-morning cartoon villain" after all.)
Also, Megatron made that offer BEFORE Optimus went to the core and the planet shutdown. Since the Autobots started leaving after the energon supply stopped, Megatron might have decided "Screw it, I'll slag them all".