Not trusting The Dozen, I accepted the quest in order to secure the weapons and avoid a potential bloodbath. Instead I found myself stuck with the faction I was trying to sabotage.
Up to that point I was given the option to betray the quest giver whenever it was logic to do so. And quest givers are not omniscient, or so many more would still be alive.
Well, there is a certain brilliance in being casually outwitted by Wenan sending the Knights in my direction, but that point in the game is somewhat messy:
Why I'm unable to warn the Knights about the plan to recover the weapons?
To fail the quest, I killed the quest giver and lowered my reputation with The Dozen to "scoundrel". Even so, I still was a "close ally of the dozen" according to the Knights Commander.
Why I can even take that quest without questions, when I openly murdered few groups from The Dozen? Someone must have seen me leave those buildings, or when I was fighting in the street. And I'm very well known. I'd expect to be told "Sorry, you are good bloke/nice lass, but I can't trust you with this delicate matter because you keep butchering my men".
Compare it to
: this one being so linear is a bit of an anomaly.