How can anyone honestly claim something so obviously telegraphed as this powerful monster is a grudge monster or a dick move?
The DM gave the players every bit of information they needed to make the correct decision. They were told the area contained monsters and that they would be out of their element. They were told -multiple times- this was the case.
And one player decided--against the group's general consensus--to cajole the team into going anyways. This is like sticking your hand in battery acid marked with a sign saying 'Warning, avoid contct; Acid.' It's not dick DMing. Dick DMing is warning the players that things will be too easy down in ther, and then pulling out Tiamat. Dick DMing is changing the next encounter to dick over a guy who says 'The DM won't send anything too hard against us.'
That isn't what happened.
What happened was: The DM gave the players correct information, and the rogue ignored correct information based on incorrect metagaming.
He ignored the game in favor of a false metagame.
That's. Bad. Play. Not bad as in rude, or bad as in unseemly, or bad as in 'That's the wrong way to play D&D' but bad as in 'unskilled' or 'prone to error' or 'the opposite of good' or 'dumb.'
It's the DM's place to create a challenging world, to enable the players to enjoy compelling stories. It is not the DM's place to make up for players being dumb.
The DM gave the players a fair opportunity to learn what was down there, a fair opportunity to avoid it, a fair opportunity to understand it might be too tough for them. He gave them all the information they required. The rogue ignored it because he belived the DM wouldn't set them up against things they could nto beat; and the rogue was right--the DM didn't set them up against this monster.
The party set themselves up against the monster.
The DM said 'This is too powerful' and the party chose to engage it anyways.
That's not dick DMing, that's giving the players exactly what they should rightfully expect.
Congratulations, you effectively ostracized a player because he/she doesn't play the way you deem fit. That's a dick move, you could have addressed this any number of ways but instead you felt he needed to be singled out and consequently now is not trusted at the table.
The rogue was being an uncooperative and selfish dolt. What about the other players, the ones who wanted to get on with the story, who trusted the DM to get them the level and treasure they needed to overcome the adversities they had plans, the ones who did not try to kill things 'for the xp' and actually immersed themselves in the fiction provided?
Do those players have to take a backseat because one wants to be a dolt? Seriously?
The rogue's actions singled him out. Everything is a direct and natural result of that. Let's not victimize the rogue here; he was sacrificing the wishes of the group for his own bad reasoning. D&D is a social and cooperative endeavor. That implies one needs to have social saavy (don't force the party into situations they know are unwise) and cooperation (sometimes, you suck it up and go with the group).
Now, you can assume that every decision a player makes is good judgment by reason of it's made by a player, but I don't think that's a universally understood group contract.
This. No amount of metagaming will ever be a good excuse to do something that is absolutely addlebrained.