Herobrine had been observing a human who was strange even as humans went for some time.
This human - who tended to share his time in Minecraftia with a certain other - seemed completely out of its mind. It seemed to enjoy screwing around with the laws of Minecraftia, trying out different ways of defying the laws of Minecraftian reality for only a few minutes at a time per method. On its own, this would have simply been strange, but for some reason, the human always seemed to enjoy ending its exploits by blowing something up! Typically, it was the human's own house that was targeted, which would have been…if not okay, then forgivable; however, a large portion of the surrounding land tended to get decimated in the process. Then, once it was over, the human would reset the world and do it all over again some other way! As if that wasn't enough, to add insult to injury, several of the events involved the human experimenting with pretend versions of Herobrine himself! Herobrine had seen mockeries of his image attack the human, be attacked by the human, kill what could only be a representation of his own brother, Notch (not that Herobrine was terribly upset at the thought of killing an avatar of his despicable brother), and be otherwise blamed for fiery or explosive catastrophes. For some reason, the human seemed to enjoy associating Herobrine's presence with a construction made of Glowstone in the shape of a large capital letter "E".
The human's often-time companion was, if anything, even more of a conundrum - albeit forgivably so, as it seemed to be simply stupid. It seemed to enjoy setting off TNT for no reason whatsoever, as though that was the solution to everything, whether there was an actual problem to solve or not. Herobrine held no grudge against the stupider human, as it seemed to simply go along with whatever the less-TNT-loving human did; however, things were getting very much out of hand, and finally, even though he had a feeling nothing much could be changed, Herobrine decided to put his foot down.
He couldn't attack the human - the human had had mockeries of Herobrine attack him more than enough times that Herobrine knew that a simple attack would be pointless. A scare campaign, however, seemed perfectly in order. Therefore, Herobrine had some Endermen construct a large Glowstone letter "E" outside the human's house, and then, the next time the human visited - coincidentally, with its idiotic companion - Herobrine made his move.
The human didn't seem interested in doing much of anything this time around; so much the better, in Herobrine's opinion. Herobrine waited for a minute, to see if the human was going to do something, but strangely, nothing happened.
The doors to the house were open. Herobrine dashed in, hoping to startle the two humans, but for some reason, they seemed oblivious to his presence, even when Herobrine ran right in their faces. Herobrine attacked the idiotic TNT-lover, but it died before it seemed to even realize what was going on. Confused, Herobrine walked around over to the other, more problematic human…who suddenly seemed to come to, and started looking around for its friend. By chance, Herobrine had walked around behind the human, so it didn't see him right away.
The human turned, saw Herobrine, froze for a second…then bolted out the door.
Naturally, it turned to its left, so it immediately saw the Glowstone "E" Herobrine had had made right outside the human's house. It turned in the other direction and ran, jumping madly, finally taking its golden sword (golden! sword!) and hacking at the ground with it, digging itself into a hole to hide.
Herobrine left, satisfied that the human had been sufficiently terrorized.
Did anything change after that?
No. Of course not.
But scaring the idiot made Herobrine feel a bit better.