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Considering that Silent Hill 3 even exists, one can immediately rule out the Bad ending. In it, we come to understand that the entire series of events was only a dying fever dream of Harry's. As he survived to later die in Silent Hill 3, this obviously cannot be the true ending.
As Harry Mason does not escape the town in Silent Hill's Bad+ ending (and, since Heather is never conceived), this ending too is clearly invalid.
Few, if any, people attempt to dispute their invalidity. But, there are still a large minority out there that believe GOOD+ is a possible ending. It certainly is not. The implication of GOOD+ is that Cybil survives the events of Silent Hill. This is impossible.
First, and most importantly, The Book of Lost Memories makes it as plain as can be:
This leaves absolutely no room for debate.The orthodox ending, which is connected to the third game
However, there is a statement made by Hiroyuki Owaku which many people believe contradicts this:
Cybil is not involved in Silent Hill 3. What happens to her afterwards is left to the player's imaginations.
I do not believe this contradicts the first statement at all, and is not suggesting that the GOOD+ ending is possible. It suggests, rather, that the fate of those drawn into the Otherworld is uncertain. We don't know that death in the Otherworld implies death in the real world, so it may be possible to die in the Otherworld yet still survive in objective reality. Therefore, even if Harry had to kill Cybil, since the Harry and Cybil we see are, in reality, no more than mere egoscopic projections, neither can actually be harmed.
If anything, the events of Silent Hill 4, where death does transcend the Otherworld's barrier, only suggest that Cybil is that much less likely to have survived, but that's neither here nor there.
However, GOOD+ has a fatal flaw that existed even before Silent Hill 3: To save Cybil, Harry is required to use Aglaophotis on her to remove her possessing parasite. The problem is, it is absolutely impossible for this to happen. Though Harry is given a subtle nudge that helps him find and collect the spilled red liquid, he never figures out its purpose or use. This is crucial: Harry cannot use the Aglaophotis on Cybil because he has absolute zero reason to logically expect that it HAS such a purpose. Harry does learn, in the end, but not until he sees Michael Kaufman use the same stuff on the Incubator. Only at this point is Harry made aware that it is a substance that exorcises demons and otherworldly parasites. Harry, obviously, is not offered this knowledge until long after he is forced to kill Cybil.
To put it simply, Harry can't utilize knowledge he does not have.
Now, the common rebuttal to this from the dissenting crowd is that Harry figures it out by accident, experiment, and/or unwillingness to kill a good friend. I submit that this makes zero logical sense whatsoever in any context.
First, Cybil is hardly a great, old friend to Harry. They have, to this point, met three times. They have spent perhaps fifteen entire minutes in each other's company. Harry might risk his life to save a life-long friend, but not a stranger who is clearly trying to kill him.
Second, Cybil is trying to kill Harry. Even if Harry wanted to experiment with items in his inventory, he's not going to do it while he has a gun pointed at him by a woman who is possessed. Even if he wished to do this for a dear old friend (which, as stated, is not the case at all), he couldn't. To experiment with random, heretofore useless items, in the vain hope that one of these things might perhaps act as a cure, is twenty times more likely to result in Harry getting wasted than anything else. This scenario is entirely illogical.
Third, Harry has already killed dozens of similarly-possessed people in Alchemilla Hospital by this point, and throughout that entire adventure he has the aglaophotis. He never attempts to save any of them with his sample . . . because he is given no hint, no clue, no sign that his mysterious red liquid will cure a toothache, much less demonic possession. To assume he comes by this knowledge before finding Cybil is assuming incorrectly. The game, again, elects not to give you this information until it is too late for you to use it as you should.
Harry would have to be precognitive, or have access to outside his own objective reality, in the form of game guides. It is the only possible method by which Harry can get this knowledge before finding Cybil. Obviously, this invalidates it within the context of the game, because Harry cannot possibly have any such foreknowledge.
GOOD+ is invalid even within the context of the first game. It is intended to be an easter egg for repeat players. It is, very obviously, not intended to be considered a serious ending to the game.
This leaves GOOD as the only ending left that makes any logical sense, both within the context of events within Silent Hill 1, and the connection it has with Silent Hill 3. There has never been a plausible denial of this, and since the developers have given GOOD its blessing, there simply cannot be any other possible ending. Unlike all other endings in Silent Hill 1, it fits seamlessly and requires no leaps of logic whatsoever.
Those of you who insist on GOOD+ being possible or even likely, are not just spitting into the wind, but ignoring logical thought processes. The ending is not possible. GOOD is the only one that works.