There was a ML said he's that much stronger than other ghosts mostly because of how old he is, since time is also a big source of power for ghosts, but the ghosts in the annex classroom (who are probably from the 1998 incident) are all weaker than him... so he's obviously much, much older than those ghosts. Another clue is that if he's supposed to be older than the school (since his tree existed before the school), and since the school was opened 50 years ago (since it's now having its 50th anniversary), then the ML must be at least 50 years old, if not more... (reading back, I've confirmed the present day of the story was stated to be the year 2008 , and that the incident 10 years before supposedly happened in 1998, so if the school opened in the 50s or 60s, the ML should be older than that).
my theory is that ml also gets some of the power the bullied kids who all become sacrifices and and drag more and more ppl and slowly drive them insane and apread the madness and all the negative energy is colleceted back into the pond with the tree just enugh to make a boundary and heesu absorbs that power

I just looked up when the first amusement park was founded in Korea. Apparently 1973, but it’s unclear if it would have been considered an amusement park. The first major theme park development was in 1989. So… it could be that they didn’t have a modern amusement park and, because he was a loner, he may not have even known about their existence. He may be just telling the truth and only be 10 years dead. Or he may be 30+ years dead. Depends on what the author considers “amusement park” to be or Hae-su’s knowledge of the world.
Either way, he’s confused. He wants to leave but he also says he’s content. He mostly wants revenge, so he’s malevolent. But he’s still carrying the loneliness of his past life so he years for real connections even though it’s impossible. Also, if this story isn’t using mostly fictional elements, he could have died from necrotizing fasciitis by having a wound in the pond or something. What a fun concept. Love the mystery aspects.
For anyone interested who is holding back because they’re worried it’ll be too scary—it’s not even close to as scary as most horror video games like Deep Space or Silent Hill. Just a bit unsettling with the teeth and eyes. I honestly think “Dreaming of the Dokkaebi” has creepier ghosts/monsters.