Ditto P-Bar in all major areas. Things you did that were not RBDMish:
1) You messed with their plan for the stated reason of messing with their plan, not because you'd decided beforehand to have an important plot twist or something.
2) You just told your players that no matter what they do in the future, if you don't like the way things are going, you're just going to drop in something to mess it all up without making a whole lot of sense. Which means that the players are less likely to show initiative and do anything inventive, because that will just lead to you doing your thing anyway.
3) You told them what you were doing. If you ARE going to do what you did, it's important to create the illusion of it being something that was going to happen from the beginning, and it was unfortunate for them but inevitable.
4) You allowed a player to create a character you couldn't handle from a game-planning standpoint (the monk). If you let someone make a massive diplomat dedicated to peace, you need to come up with reasonable ways to a) let him use his abilities and feel good and b) get around his ability so that combats still happen often enough to be fun for the group. What you did is a bit like letting a sorcerer take Invisibility, getting frustrated as he walks through the camp undetected, and then ruling near the end that the shaman has an amulet that lets him see invisible creatures, although that amulet won't work for the party if they get it after the fight.
Not RBDM. Not evil DM or bad DM or anything like that either, though. I think you can solve the situation with better planning and a bit more flexibility with regards to your adventures.
1) You messed with their plan for the stated reason of messing with their plan, not because you'd decided beforehand to have an important plot twist or something.
2) You just told your players that no matter what they do in the future, if you don't like the way things are going, you're just going to drop in something to mess it all up without making a whole lot of sense. Which means that the players are less likely to show initiative and do anything inventive, because that will just lead to you doing your thing anyway.
3) You told them what you were doing. If you ARE going to do what you did, it's important to create the illusion of it being something that was going to happen from the beginning, and it was unfortunate for them but inevitable.
4) You allowed a player to create a character you couldn't handle from a game-planning standpoint (the monk). If you let someone make a massive diplomat dedicated to peace, you need to come up with reasonable ways to a) let him use his abilities and feel good and b) get around his ability so that combats still happen often enough to be fun for the group. What you did is a bit like letting a sorcerer take Invisibility, getting frustrated as he walks through the camp undetected, and then ruling near the end that the shaman has an amulet that lets him see invisible creatures, although that amulet won't work for the party if they get it after the fight.
Not RBDM. Not evil DM or bad DM or anything like that either, though. I think you can solve the situation with better planning and a bit more flexibility with regards to your adventures.