(Set in my Jazz Prime AU--aka my Weird Time Travel AU, lol)
For a moment, no longer than a sparkbeat, he couldn't quite process what had happened. Hot Rod had come out of nowhere--Optimus hadn't even realized the young bot was close enough to come to his aid--and was attempting to wrestle a hidden pistol out of Megatron's hand.
The moment of shock passed, though, and the Prime took a step toward the pair. Then another. Time slowed down strangely, everything around him feeling suddently surreal as Hot Rod lost his grip on the pistol.
He took another step as Megatron lowered the weapon. A fourth as the pistol was pointed at Hot Rod's chest plates.
On the fifth, Megatron fired.
"NO!" The Matrix howled with him as the blast tore into the young mech's plating. Megatron fired again, shredding Hot Rod's chest cavity and demolishing both spark and laser core.
Hot Rod was grey before he fell to the ground.
"No more, Megatron." The Matrix mourned in his chest, its grief like nothing he had felt before. Spurred on by its anguish, Optimus activated his energon ax and charged the remaining feet between himself and his old enemy. "This ends today!"
"As you wish, Prime." Megatron's face was cold as he re-aimed the pistol.
Optimus swung the axe downward as Megatron fired. The edge of the blade cut into a silver helm as the energy blast slammed into red chest plates. The Decepticon fired again as the Autobot twisted his blade to increase the damage to the warlord's processor.
The Matrix flared as its carrier's energon pump was damaged.
Megatron's pistol fell from unfeeling fingers, plating fading to grey as the Prime's axe jerked forward and severed a primary neural connection.
Optimus fell slowly, collapsing over the top of the bodies of his young protege and his old enemy. Energon dripped onto the ground under them.
The Matrix of Leadership flared again, grief the Prime didn't understand overwhelming him and sending him into darkness.