Somewhere else in A4 there is a valve that turning will kill every single npc in arx. Do you also think this is a bad design?
No, because the valve and the consequences for turning it are clearly indicated at the time you can turn it. That is not the case here. There is a difference between "do this and you will definitely kill pretty much everyone" and "do this and you will have no way of knowing who dies". AND once again, in this case, I did not make the choice.
EDIT: I'll also mention that I was totally on board with this whole part right up until I realized that I'd need to go back several hours, redo the boss fight, and re-do all the shopping for level 21 gear.
The dilemma worked for me, it was effective, it was already doing its job. Screwing over players is absolutely not necessary for the sequence to have the intended effect.
There's also need to have only one vendor for the books in the act. Add redundant vendors and I would be back on board with this.