Disclaimer: I live in the US, not Great Britain, and am not JK Rowling. Therefore, I don't own Harry Potter
Being a ghost, to put it simply, sucked. Or it was bloody hell, take your pick.
Until he'd died, he'd never considered becoming a ghost. Eternal life was great and all (unless you listened to the Wing, who insisted that eternity got very boring, very quickly, which was the only reason they were in England at all), but eternity as a ghost would be…well, hell.
Truthfully, the only reason he decided to at all was to go and give his friend a good kick up the arse before telling him that he wasn't at all to blame and he really needed to get out of the castle and into the sunlight before people started thinking he was a vampire (something that certainly would have damaged what little reputation the school had so far managed to build).
But the application took a lot longer than he thought it would, and by the time he completed the paperwork, his friends and coworkers had already passed on and the Wing had left. Consequently, there was neither a single friendly face within the castle, nor anyone who would believe he was who he said he was.
His appearance might have something to do with that. His death hadn't been slow and agonizingly drawn out, but it hadn't been instantaneous either, and it had left him rather a mess, and that didn't change when he became a ghost. Yet another thing to complain about…bloodstains were hard to get out of clothing in life; it was impossible to do so in death. You don't picture a legend worn out and covered in blood, after all.
He doesn't remember when, exactly, "Professor Slytherin"—however mockingly it was said—became "the Bloody Baron." He just knows it was before the other ghosts arrived, and probably originated with Peeves. Bloody poltergeist.
Really, it was no surprise that he sunk into doldrums. He couldn't argue with Godric anymore, or badger the women until even Helga lost her patience with him, or duel with the Wing; neither could he pursue his studies in magic in ghostly form. Before too long, he no longer noticed the passing of the years; Headmasters and classes came and went without making any impact on his stagnating brain. Not even his own descendants managed to penetrate the fog.
They probably would have followed his son's ideals at any rate. Bloody fool he was, to take Salazar's dislike of the Muggleborn students and turn it into hatred. Salazar had been glad when he'd left.
Not even the opening of the Chamber of Secrets affected him, other than that another ghost joined the gathering in the castle. He vaguely noticed her—a weeping, wailing girl, apparently named Myrtle, who took to haunting the second-floor girl's bathroom—but of the events that led to her death, he knew nothing, and he preferred it that way. Let him ponder over the past for eternity; he wanted no part of the present.
Things had remained the same for Salazar Slytherin for a thousand years; now, they were about to change.
This story grew out of a comment made by Nearly Headless Nick in Anne Walsh's Dangerverse; I can't remember which story or what the actual quote was, but Nick mentioned that the Bloody Baron was the oldest ghost in the castle and had actually died there. That made me wonder...what if the Bloody Baron is actually Salazar Slytherin...