Dearly

The problem wasn't that Lumielle was a loose canon that might attack to kill Aerin. But she would attack to kill Lyra, that was well within her degrees of tolerance. 

The first problem with that was what it meant for her to kill Lyra in the first place. She would certainly leave the Dwarven Bubble, and certainly expose herself. Then she would be putting herself directly in the line of danger on top of that where the Spirituals would have full rights to kill her if it ever came to that. 

But that was only the first problem. What kind of strife would this cause to the Dwarven Race? 

Even if Aerin didn't know the extent of Lumielle's true power, he did know how important she was to the Race. Everyone placed both of them on the same level. 

If she ended up rampaging because of this, many would take her side. And if she ended up dying because of Aerin's actions, the result would be even worse. It would cause a fissure in the Race that couldn't be solved.