Harold was hell-bent on eliminating the presence of the Transformers on Earth and considered Yeager and his friends, family and acquaintances to be traitors to the human race for siding with the Autobots. However, he himself worked with the Transformer Lockdown, and showed no concern about human collateral damage when KSI transformer Galvatron went rogue despite defending his actions by claiming that he sought to "protect" humanity from the alien war.
Harold mercilessly stalked human mechanic Cade Yeager and his daughter Tessa, due to their allegiance to the Autobots and worked alongside bounty hunter Lockdown who gave Harold given the Seed, a Cybertronian bomb built by the Creators that able to turn the surrounding landscape into Transformium. After Optimus Prime was captured, Attinger attempted to give the Seed to Joyce in return for his payment, but Joyce left, knowing that Galvatron wanted that Seed to rebuild his army. Later, Galvatron retreated as his army was destroyed by Lockdown returned to fight Optimus. Attinger notified Lockdown of the situation, the location of the Autobots and the Seed they had stolen. He cornered Cade with a gun while Optimus fought Lockdown and he ranted about how it takes patience to make a man and to work in the shadows to "protect" all humans. Harold claimed that there were no good aliens or bad aliens, just the humans and them. He said to Yeager, "You chose them" whilst on the trigger, but was quickly shot in the chest by Optimus, killing him on impact and finally avenging the deaths of the Autobots and humans he had killed.