First of, no spamming the comments with "Herobrine's not real". I never claim he's real, I just state my opinion on the origins of the figure in Minecraft world.
Also please don't bother with comments on the Cannon, Cannons are for Pachelbel

1: Notch and Herobrine
"After the apocalypse that left most of humanity dead, infected, pathetic mutations known as Testificates, or all three, one small group of survivors remain. The immune survivors were led by the brave hero Brine, and his twin brother the wise Notch (don't ask me who named them). The survivors spent their days building mine shafts, gathering food, punching wood, and enjoying their "Dragon free" world until Brine noticed some odd properties of the flight patterns of ender pearls. he negated the teleporting effects with blaze powder and discovered that it always flew in one direction, no matter how he threw it, what was in the way, or what magnets he put in place. He made more, told the others about his discovery and gathered the greatest warriors to set out to discover what it led to. After many nights traversing, unconquered terrain, many throws of the "Eye of Ender" and many casualties, the team found a cave. They explored it with Brine, Notch, and five others still alive. At the end of the cave, there was a strange brick that was certainly not natural. The broke it and made their way through the system of chambers and tunnels. They finally made it to a room with runic carvings on the wall and a circular structure suspended over lava in the centre. They saw the eyes of ender fit perfectly in the holes on the structure, so Brine set to work placing them. On the last one at the far side, as Brine was balanced over the lava, a long arm came out from the shadows and shoved him. Notch threw his sword at the Enderman, but it just disappeared. Notch was to late, but he made out four words from his brother before he died. "Sort of dragon free". Notch now knew what the runes meant, and what he had to do. He carved into the stone walls "Here Lies the Hero Brine". Then he placed the last eye, and a portal appeared. Notch then forced the others to go back and care for the other survivors while he alone killed the Dragon that awaited on the other side of the portal. Notch entered into a world with a void for a sky, yellow-green stone for dirt, Endermen everywhere, pillars of pure obsidian dotting the landscape, and the Ender Dragon flying above. He fought an epic battle, destroying the beacons that healed the Dragon and throwing snowballs at the Dragon itself (It worked didn't it?). He killed the Ender beast and it detonated with a flash of blinding light. It released orbs of light that pulled themselves toward Notch, giving him power. So many orbs, and so much power, Notch was elevated. He became the ultimate source of good int the world, he became immortal, he was a god. But every source of good must be contained by a balancing source of Evil, and so the council of Ender that was ultimately responsible for more than the survivors knew, resurrected Brine, not as the noble hero he used to be, but a crazed shell called Herobrine (Endermen don't know English very well), with a purpose of true Evil and god-like abilities to balance out Notch, and with eyes that glowed white. Notch went back to the camp of survivors, but found nothing but the corpses of his friends and family. He knew the twisted demon that took the skin of his brother was to blame. Notch knew it would have to balance out, but never even thought that how great his young son would become, or that the creature that was brought back with the son could be. Notch then knew that in order to keep Herobrine from interfering with the child's journey too much, he must also not interfere. Notch left for the far lands, where he would live out his days watching his son but interacting only through noise when caves are near, however this draws Herobrine closer.

P.S. The death of Brine, the first death of the Ender Dragon, and his second slaying that is yet to come don't yield a scale unbalance because they all died naturally (not by divine interference). So the heavens have nothing to do with them, therefore they can't balance them.

2: Abilities
Herobrine and Notch are pretty much Smith and Neo. They can fly, withstand tons of damage, but they are very mortal. Herobrine does not walk, but motionlessly glide across the floor. Herobrine has the ability to possess creatures, cause delusions, Teleport (Like an Enderman!), Instantly destroy blocks, create enormous structures, and from the time it takes him apparently access MCEdit. For some reason Herobrine can only speak backwards. I don't know why.

3: Intentions
Although he hunts alone, Herobrine still works with the Endermen, so being somwhat divine he can't kill Notch's son (who called himself Steve because he wanted a normal name to make him feel less alone.) or else the Ender Dragon will die because it was recreated from Steve's life. Herobrine can, however, drive him nutty as a fruitcake. When he discovers Steve he'll mostly just stalk him, place diamonds in wrong chests, fill his house with sheep, lock him out of his house, etc. After a month or two of that, Herobrine will pick it up a bit and start lighting small things on fire, stealing valuables, spawning monsters in Steve's house, etc. The third stage begins after a few weeks of that, and starts to get Steve's screws a little loose. Steve starts to find entire supply Testificate Villages filled with severed limbs, his cat and dog decapitated and gutted, and his house burning down in front of him if it's wood, or rigged with a TNT trap if not. Eventually Herobrine will drive Steve absolutely out to lunch for crackers and bananas, then hopefully get him to kill himself or get himself killed. This method ensures no divine intervention and more entertainment for the Angel type Herobrine. If this fails, his hatred for Steve is stronger than his loyalty to the Counsel of Ender, and he may kill Steve himself.

This is my story of Herobrine, and why he has it out for Steve.