I agree with you to some extend. However, it is about the oath to the DO. If it was that easy to ignore the oath, anybody could become a darkfriend on paper and help the light. What's worse, chanelers appear to swear on an oath rod, so they cannot simply change their behaviour from one moment to the next like Ingtar did. Like standard AS cannot tell a lie. Verin was very specific about that: only in the hour of your death may you betray HIM. Which to me indicates that she could not pretend to do things in his name when her goals were different.