Although Hal seems friendly, he is, in fact, the last of his species left alive. Hal follows Wall-E in the hope that one day he would stumble across other living creatures, so then his purpose could be revealed.
Back in the days not long after humans had left or perished, and all other animals except Hal had died, Hal had unknowingly stumbled upon a field of nuclear waste. There he found the bodies of other cockroaches who had passed on, not too long ago. Driven by hunger, Hal consumed the corpses his dead brethren. Slowly, unbeknown to Hal, changes started to take place within his body. He became stronger, more resilient even than he was before, prolonging his life. His bloodstream was altered, making him a host of a viral pathogen to which he was immune. Finally, his mind became more aware, attaining a higher standard of thought than even the humans. He had knowledge of the events that had come to pass, and could gain a distant, blurry view of what was to come.
Retreating deep into himself, Hal searched with his newfound vision, and gained much knowledge of what was to come. It was then, therefore, that he set out on a trek of many thousands of miles, aided by his newfound strength and speed, to find the robot that his mind's eye had seen. The trek was not quick, but time was something that Hal had in abundance. Finally, after much hardship and toil, Hal reached Wall-E, who at this time was but a mindless robot, the last of his kind still functioning, going about his useless attempt to clean the planet. It was Hal that awoke Wall-E's mind, who gave him self-awareness. It was Hal who, giving up a small part of himself, gave Wall-E what was necessary to carry out the task that only Hal had any concept of.
So it was for many a year with Wall-E. Hal waited patiently for the arrival of the being which would be the spark to begin the fire of his victory, and for the last remaining plant to be revealed. It was in these times that Hal brooded ever more, his eye searching the future, that he might change it. He sat inside his Kremie rolls, contemplating the end of all things, while also savouring the flavour or the delicate morsel his robot counterpart had humbly gifted him with. Though Hal no longer required food for sustenance, he had taken a liking to Kremies and had decided to one day return them to mass production upon the Earth.
Now it came to pass that the time had come, and Wall-E, unknowingly, came upon The Plant. Directed by Hal, Wall-E took the plant and left it in Hal's possession. It was not long afterwards that the being, EVE, arrived. Wall-E, foolishly, tried to establish contact with her. After his servant's unsuccessful attempt Hal took matters into his own hands. He approached the being and spoke. The being immediately fired her plasma ion cannon at Hal. Of course, the cannon had no effect upon Hal's body, and from the crater he rose. The being was confused, and Hal took the opportunity to make physical contact, all that was needed to seal the process that he had long ago performed on Wall-E. EVE gained self-awareness and from that moment forward the inevitable gears began to spin, slowly onward to their inevitable end.
Wall-E showed EVE The Plant, as fate had intended, and EVE produced her inescapable reaction. Soon, EVE's spacecraft returned to take her, and Wall-E stowed along, as Hal had intended. Hal was more than happy to remain on Earth as he awaited his servants' return... with their precious cargo.
Before long they had returned, more successful than Hal had dreamed. Humans flooded from the starship and began their new lives. Hal was eager to reveal his purpose, but he knew the time was not yet ripe. Hal watched as humans spread upon the Earth, time dragged on, year after year. Then, time. Hal removed his power from Wall-E and Eve, rendering them mindless robots once more. The humans were shocked, and mourned as such, trying everything in their might to return them to their pre-vegetative state. There was nothing to be done but to shut them down, and give them a proper burial. The humans built a monument to the robots, and it was here, from the top of the monument, that Hal made his move.
It is the day of the funeral, and the human population, now numbering millions, has gathered around to mourn their mysteriously departed friends. Eulogies are read, a particularly moving one from the aging former captain, now chancellor, stirs many in the crowd to tears. No one notices the cockroach perched high on the monument, now emanating a thin black gas from his body. The gas disperses into the air spreading unnoticed over the crowd.
John stands at the base of the statue, tears in his eyes as he reminisces over the times he shared with Wall-E and EVE. Unnoticed by him was the change occurring within his very cells. The packages of DNA, mitochondria and organelles that made up his body were changing, though not, for now, ceasing their primary function, they were morphing, expanding. Before long he was itching, though in his grief this went largely unnoticed. Then suddenly, without warning, he burst. Each of his cells had suddenly and irreversibly morphed, each or them forming into baby cockroaches. Around him, countless millions gasped... before they too were consumed.