My understanding is, she's trying to release the negative energy of the undead, their suffering and unhappiness, at a point, with the twin intentions of freeing them from undead-ness and weakening the Veil at that location to permit passage. This assumes two things: First, that doing so will "free" the souls, and second (possibly more important) that she can then control that energy, contain it, and dissipate it afterwards without it hurting anyone.
Oh, also that channeling all that energy won't have unfortunate repercussions on her own soul in the process.