I can't be so categorically against the OP, even though I don't agree with him completely either.
If you are promised to kill a boss, and that was your reason for joining you are getting shafted and should be annoyed. If the others hadn't confirmed with the leader before hand that princess would be saved I'd say that the group was morally bound to killing her even if the majority wanted it the other way around based on a simple "a deal is a deal" logic.
In this case though, looking at the screenshots. It does seem like the group was actually formed as a save princess run, and that Light was just oblivious to the fact that he created a group with mutually exclusive goals. Not a good thing to do, and yes as the leader you are in fact the one to blame.
Then again, the crime is minor and Light doesn't deserve to be crucified on the forums for it. Rather just told that you should stick to your arrangements or people will not trust you. In WoW as well as in real life. And the OP is well to learn that when you get shafted, you generally will not get any compensation for it, in WoW as well as in real life.