Powerful and dangerous? In what way was Aran'gar either? She had just lost her power base, and was almost certainly in bad odor with the DO and Moridin for letting Egwene get out of her control. There's no way she'd compound it by harming Graendal, who stands second only to Moridin in the DO's eyes.

And Aran'gar is a fool, easily manipulated. Graendal's pets keep her distracted well enough, as Graendal implies.

The only reason you object to it is because your pet idea that Graendal is dead and hence not an Asmo suspect is now off the table.