"Please, Marty. It's okay" said Doc.
"Okay my ass!" yelled Marty. "You deprived the world of infinite, free, clean energy, in exchange for selfie sticks!"
"I made sure humanity survives 2045." replied Doc.
Marty approached him. "I remember a man I used to know" he said, pacing around him. "He told me that the problem is never technology, it's always people. What's left in you of that man?"
"I fail to see your point." said Doc.
"My point is" hissed Marty, "that if in 2045 Griff Tannen blows up fusion generators, you are not supposed to remove fusion generators from the timeline! You are supposed to remove Griff Tannen from the timeline!"
"It's all in the past." said Doc.
"Stop saying words, Emmett." retorted Marty, pacing and looking around the lab. "It is now clear to me that you are the real Destroyer of the World, and I will do whatever it takes to stop you. Because, you see, the problem is never technology. It's always people."
Marty set his eyes on the work desk nearby. ROHS-compliant solder, copper cables, a soldering iron, a box with the radioactivity symbol, a hammer, a set of screwdrivers, a set of wrenches.
That wrench.
Chromium-vanadium tool steel.
Sturdy.
Heavy.
Marty grabbed the wrench and ran toward Doc, with a scream of desperation.
Jennifer intercepted him and grabbed his arm from behind. He turned and hit her in the middle of the chest. "You, stay out of this!" he yelled.
With Jennifer on the ground, writhing in pain, Marty swung the wrench at Doc, who ducked and dodged the blow. The wrench smashed through a cabinet full of chemicals.
Marty looked at one of the bottles. Sulfuric acid. He grabbed it and threw it at Doc.
The bottle shattered on the floor, less that two feet away from Doc, who stepped away from the acid. This brought him within Marty's reach.
With a final swing, Marty struck hard against Doc's nape. Metal met bone, and bone lost.
Doc lay on the floor, motionless.
Marty placed his fingers on the old scientist's neck. No pulse.
Behind, he heard Jennifer's voice. "Marty, please, end this madness. I'm hurt!" she said.
"It will be over soon." he replied. "Soon we'll be home, with two teenage kids named Marty Junior and Marlene. We'll have flying cars in our garage, I won't have Parkinson's anymore, my parents will still be alive, kids will roam the streets with hoverboards and everyone will laugh at video games where you have to use your hands."
Marty grabbed the plutonium case, put on a radiation suit and inserted a charge into the reactor. He placed the case into the trunk, entered the time machine and set the destination time: March 7, 1989.
"Three days. Should be enough to reach the University of Utah." he said to himself.
He closed the gullwing door, drove out of the lab and accelerated to 88 miles per hour. The Delorean disappeared in a blinding flash of light, leaving two trails of fire behind.