I suppose I'm gonna be the Devil's Advocate on this one! (BTW Before I begin, Kudos to your DM. Anything which makes you think about your character is a good scene!)
I don't think you did the right thing. It certainly wasn't good, though I wouldn't put it at evil. Killing people who are about to wrongly order your death (at least, you felt being chained was as good as a death sentence here) isn't evil, it's self-serving (and thus neutral).
However, I assume you are a reasonably powerful Sorceress (at least 6th level to be throwing fireballs around), travelling with two Paladins who can look after themselves, and a spell-casting, shapechanging druid. You know a number of things:
1. There is a murderer who wants you chained so he can come and kill you.
2. The captain is going to take away your items and chain you and lock you in a room somewhere.
3. You do have allies who are aware of your feelings and who could come to your aid, even if only as support or a distraction. (The druid's companion)
You can plan for the event, of the killer coming for you. You can be prepared for the killer, and even bound two Paladins, a druid and a Sorcerer might well have been able to overcome the challenge, maybe even break the chains or pick the locks? Shapechange into something small, then into something really big? Equally, you could have asked the captain for a guard to protect you, at least buying you a little time. This would have smoked the killer out and allowed you to unmask him/her/it.
Fireballing the room might have seemed like a good idea (in case there was someone really dangerous in there, but if they were that dangerous, and the fireball didn't kill them, doesn't that mean you're in big trouble anyway? And you've just legitimised your deaths; they don't even need to sneakily bump you off now, you've turned everyone against you. It may have been the only option, but I think it was probably the wrong decision.
As for the Paladins, I think you could be forgiven if you were remorseful about it. They shouldn't be overly happy about you fireballing people who are effectively doing the right thing.