Nice Exit 8 reference.
Another chapter, another Wall of Text.
Hai, it's me, the Voice of the Contrarian.
I won't be quotting post by post because it would take forever and would make the comment even bigger, so I'll just quote some general ideas I have seem a lot in this comment section:
Miko's father and the horse
First of all, I'll once again say that
I highly doubt Miko's father had any kind of powers before his death (or after), or that
the horse is anything more than a random ghost that took a liking to Miko's father and thus protects him, and only him;
"Why Miko doesn't try talking to her father?"
At least one other person mentioned it already, but even if she knows her dad isn't going to attack her,
she sure as hell won't be trusting the horse anytime soon;
"Seto-san says he's on humanity's side but have shadow people around him!"
Ever heard of "Fighting Fire with Fire"? Again, we still don't know if they are some kind of Shikigami or something, but his MIB underlings are obviously useful, only a fool would get rid of them just to "show which side he's in";
Have seen quite a few people saying that he's right, which sure, ghosts obviously don't belong in the living world, but there are ways and ways of doing things, it may or may not apply to this story, but in Ghost Hunt, Masako mentions how there are 2 ways of exorcising ghosts: Help them to move on, or essentially destroy them (which to her, who was always able to see ghosts, destroying them equated to murder). Which's to say, destroy them is cruel. Of course, there are many that are too far gone and that would be the only way, but that's no way to deal with a frightened child.
Now, what he said can be interpreted as an implication that in the past he used to make sure a ghost was malicious before destroying them, but after many many years in the business, he got tired or something, and decided to take the "easy path" of just destroying them all without concern, and no matter how right he is in the grand scheme of things, he's still causing unnecessary suffering to non-malicious ghosts (and possibly erasing them from existence), and that distinction will (probably) put him in the "wrong side";
Side note: Saw that some tried to advocate for the child by saying that "the harm she did to Seto-san's co-worker was unintentional", but let's not overlook that considering what Seto said, he doesn't actually know if she was even the culprit, he just destroyed her because she was there.
Again, some others already explained it, but what influences a ghost's appearance is emotion + time, all ghosts will get corrupted with time, that's because they eventually forget why they are there (a.k.a. purpose), and even who they are (identity), that with being susceptible to the emotions around them, specially negative ones, and you end up with inhuman monstrosities (needless to say, if you were already evil in life, becoming a moster in death becomes even quicker). On the other end of things, there are those we have seen that are just waiting and watching over their loved ones, so in their case they have a clear and strong purpose and a strong emotion that serves as an anchor for their sense of self.
Some notable mentions I have seem in a couple of comments:
The boy that was evil but seemed like a normal boy. In his case, it probably hasn't been long since his death, but he was increasingly swallowed by a sense of abandonment, which made him "evil" (mostly liked wanting people to suffer because he's suffering) before corrupting his form, but that would eventually still happen (if I remember correctly, his arms become pretty long when he's about to attack, so it was already happening);
The ghost guy in the train that hunts other ghosts. Of course I can't go and say that I have a perfect explanation for that one, but there's two easy hypothesis, 1. He died because of ghost, and now has a grudge on other ghosts, hence why he hunts them; or 2. He was some kind of vigilante in life, and after death, despite the corruption of time, his sense of duty is still strong enough to keep him hunting for "evil".
Like last time, I'm sure there was more to say, but I forgot, plus, I already rambled enough.
The explanation of ghost's nature may not be the best and possibly even confusing, if that end up being the case, I apologise.
Either way, Wall of Text, over.