I was wondering when you'd get to that question. Check out the mantle over your shoulder. The insignia is that of the Harpers. We are an independent organization of adventurers and others who fight against evil organizations throughout Faerun. You see, quite simply, there's far too much evil in the land for the governments like Cormyr and the major adventurers to keep in check, which is where we come in. We do whatever needs be done, and we answer to no one. Sometimes, we might bend the law, sometimes we break it, but it is for the greater good and so that the laws of good may survive. We work against evil cults, pirates, slavers, whoever happens to be stirring up trouble. And if there's one thing the Fire Knives excel at, it's stirring things up.
To answer your question, I did what I did because the only other ways it would have ended for you would have been on the headsman's block or at the business end of a Fire Knives' blade, and neither of those would have stopped the Fire Knives from trying again. They may succeed the next time, and then they'll go after the Crownsilvers, the Huntsilvers and the Truesilvers, and then Tagreth Cormaeril would be on the throne. I will do whatever is necessary to ensure that day never comes to pass, for if it does, Cormyr will most certainly fall to evil and darkness.
[sblock=correction]The three sublevels in the sewers are 1000 (one thousand) yards long, not 100.[/sblock]