That is beside the point.
The fact is, here is an entity that says - though its proxies, but still - "I'm going to give you powerful secret knowledge if you let me control you." You can be sure that this entity, like everyone else, follows her own agenda rather than yours, and you can't be sure whatever she'll force you to do eventually will be anything you'd even remotely agree to if you had still a choice. The situation is the same with any spirit of an unknown type. The situation would be the same with any human of unknown disposition, were they in a position to do the same.
Essentially you are right.
However, a Dalish Elf might disagree with you out of religious belief, he or she will offer themselves willingly to the goddess,
perhaps partially in a hope to use this priceless ancient knowledge to improve the state of the Dalish in general.
There is also something else to consider: Abelas (sp?) or the others before him that were under the control of the Goddess - are not
known for going on rampages or starting holy wars in the name of their goddess. This might imply that the goddess is not interested in the kind of acts you fear.
The last point to consider is risk Vs. reward: What a garden variety demon might offer to a Mage are mere parlor tricks and trinkets in comparison to the power of the well of sorrows. Can you put a price on this kind of knowledge? Is your own complete freedom worth more to you than what the well has to offer - especially when the price is very vague and seemingly not at all as dangerous as a regular possesion?