Your DM is making a mistake in hiding that he is possessed, and it is causing you problems and, I suspect, causing the opposite effect of what they want.
The first part is obvious: the DM has put you in a position to make some great RPGs, but the rest of the group was not ready for this kind of thing to happen.
In my experience, campaigns in which the actions of a character can be influenced by things other than the normal role function should establish this. I left the Abyss, which plays a lot with the mechanic of madness, and caused my players to develop all kinds of fun and strange quirks and defects. But, I warned you that this could happen beforehand. The characters did not know, but the players really enjoyed it.
The DM needs to defend it and explain what has happened, to preserve peace, if nothing else. This will require everyone to adequately commit to not get into the machine with this information, but that should be standard at any table anyway.
The second part, however, is where I finally put my liberal arts title to good use. The DM goes by surprise, but they are doing it at the expense of suspense. Alfred Hitchcock said it best in an interview with the French film critic, François Truffaut:
There is a clear difference between "suspense" and "surprise" and, nevertheless, many images confuse the two. I will explain what I want to say.
Now we are having a very innocent talk. Suppose there is a bomb under this table between us. Nothing happens, and suddenly, "Boom!" There is an explosion. The audience is surprised, but before this surprise, he has seen an absolutely ordinary scene, without any special consequences. Now, let's take a suspense situation. The bomb is under the table and the public knows it, probably because they have seen the anarchist place it there. The public knows that the bomb will explode at one o'clock and there is a clock in the decoration. The public can see that it is a room to one. In these conditions, the same innocuous conversation becomes fascinating because the public is participating in the scene. The audience longs to warn the characters on the screen: "You should not be talking about such trivial matters, there is a bomb under you and it is about to explode!"
In the first case we have given the public fifteen seconds of surprise at the moment of the explosion. In the second we have provided fifteen minutes of suspense. The conclusion is that whenever possible the public should be informed. Except when the surprise is a turn, that is, when the unexpected ending is, in itself, the climax of the story.
If the rest of the players Knowing that there is a bomb under the table (a member of the group possessed), gives everyone the opportunity to flex their role playing muscles, and can create an excellent suspense, instead of just a brief "gotcha!" Moment after a long time the party gets angry with you.
A quick warning
All this means that your table focuses on the collaborative narrative instead of a story curated and told by the DM. This will involve a lot of effort on everyone's part, but if everyone is willing to do it, I think this could be a really exciting turn.
Well, not a ton, if your DM is not willing to listen. I would send them Hitchcock's appointment, since he has a bit more authority than me, a random poster with a Garfield icon. I would highlight the discord that is causing among the players, and that this tension can be difficult to solve. Send them a link to the problematic players tag here, if they do not believe you; If there is no explanation in the game for this behavior, the number one tip is to talk to the player and then let him go. (Or recommend the annoying player to leave, if the DM does not take care of it). You are not doing anything wrong, but the way your colleagues perceive it will not end well. the match You should feel betrayed by this, not by the players.