Because, clearly, we know what defenses the General has in place? We know what agreements or compulsions the dragon may be stuck with? Could not a halfway decent GM not come up with a half-dozen reasons for the dragon to not want to directly attack, that the PCs wouldn't know about, in as many minutes? Sheesh!
I have a different take on this altogether. What I see here seems to have little to do with character alignment. It has to do with player expectations. Some players felt this was a game where problems would be generally solved by fighting. The OP thought it was a game where finding non-combat solutions was expected.
Expectation-mismatch. Fix? Talk like reasonable adults about what's expected in the game - "Hey, GM, did you really expect us to fight our way through that?"
Absolutely.
But my initial assessment is that placing the dragon at the scene, the GM was overcomplicating it and setting up a railroad. When a first level party meets a dragon capable of carrying them, if he doesn't wipe them out, it's because the GM is going to make them do what the dragon wants. Because the party cannot beat the dragon, and at best, can run away.
Since the dragon carried the OP to the general, I kind of get the feeling the whole thing was a setup for that. Any reason the dragon couldn't do it himself was arbitrary. It's entirerly probable the GM didn't want/expect the players to kill the cultists, as his whole goal was likely to reverse the players course and get them to kill the cultist.
Obviously, we'll never know. But if the players talk, they can better understand why it got goofy because of how each player looked at the situation. Each player needed to recognize that the GM was doing something screwy and they may be hosing themselves. The General may have been a bad guy actually.
The OP needed to assess whether his new information was actually information. Was the General a truly a good guy, or did the party even know? If yes, then was the Dragon a bad guy? Usually, yes. If so, then anything the dragon says is a lie. A big monster on the scene of a low level party is meant to force the party down a path. The OP was already intimidated enough to know he couldn't fight it. I'm surprised the party was dumb enough to keep fighting in its presence.
My point, to the OP, is that with his own viewpoint laden version of events, I can find logical cause to side with the other players plan to stick with the general. Whether I'm right or not, there's a reasonable logical chain to support their viewpoint. Which means, they weren't on crack, nor were you. The situation was complicated and it put party unity at risk.
As a GM, I find more fault with the GM. Some GMs like to mix it up or be contrary, but I find it serves little good to add ambiguity to the first adventure. Make it straight-forward so you can get the players to bond as a team. Don't bring in high level NPCs to bully the party and make them choose the right path because they can't oppose the high level PCs.
the GM may have revealed a point that "you shouldn't blindly follow the first plot hook you're handed", but he did so at the expense of the first adventure. There may be some bad blood between players now. Maybe that can be fixed.