Megatron was never one to let his own troops harm him, but apparently Starscream had gotten the best of him and left the old warlord to die.

It was only when you, an Autobot soldier, picked up his signal and headed towards him. You never expected to see Megatron there, bleeding out in the woods.

Even if you were an Autobot, you never could let someone die, even if it was Megatron, a tyrant with no hope left for him. It pained you to see someone die.