They deleted him from the system before they gave the order for him to be killed. It was a mistake that they shouldn't have been able to make, but once they'd made it, it was impossible for them to track it down. They had no way to stand outside the system and review it, and they didn't know about the system logs. The man who did, was the man they'd deleted and then ordered killed.
The robo-delenda awoke in section twelve and checked their orders. They downloaded the serial number for the terminee, selected weapons, and rolled out of their cubicles. His tracking beacon relayed its position on request, and they closed in. He was in a hospital.
The waiting room was almost empty, there was one man sat in a chair, looking relaxed but alert. The robo-delenda ignored him, rolled past, and through the double doors, still looking for their target. Their target, the man in the waiting room, smiled now, aware that his modifications to the system had had their intended effect, and had gone undetected. He stood up, left the hospital, and crossed the plaza outside to a skyscraper office building. He paused at the door; now that he was deleted from the system he had no access rights. He pressed the door entry button anyway, and waited while his retinas were scanned.
Inside the hospital, the robo-delenda located their new target, the person who'd been given the serial number previously belonging to the man. The baby was two days old, and being tended by nurses who did their best to save its life. The robo-delenda were unstoppable.
The doors opened. The access button had, the system believed, been pressed by no-one, since there was no match for the retina pattern. Therefore it was safe to open the doors. The man went inside, and found the lifts.
An alarm was flagged up in the system. No child under the age of five could be scheduled for deletion, yet two robo-delenda had just deleted such a child. The only cause could be corruption in their programming.
Still in the hospital nursery ward, the robo-delenda opened fire on each other, new orders coming in.
At the top of the building was an unsecured terminal with system access. Something else that was supposed not to exist, but this building was only accessible by the very-highly trusted, usually escorted by robo-delenda in case that trust was misplaced. The man sat down.
The system registered that more children were dying in the fire-fight and sent auto-destruct sequences to the robo-delenda. The explosion destroyed the entire hospital.
The man looked out of the window at the burning wreckage, and then rested his fingers on the keyboard. For a moment he wondered if he was doing the right thing, and then he decided he didn't care any more. Society shouldn't be ruled by robots.
He typed "exit" on the keyboard and tapped the enter key.