Merril's word is irrelevant in that context. Demons always call other "evil spirits" demons. And they always call the "good spirits" spirits. If it was the Dalish camp then yes spirits can called both demon and spirit. In the fade there is a clear definition and difference between the two. Justice calls Torpor a demonic monster while Torpor calls Justice tiresome spirit. Its obvious really.
Also this:
As Justice points out, the denizens of the Fade are influenced by mortals. The distinctions held by Andrastians are shared by some of them, like Justice, who doesn't even recognize what the Dalish elves are, and requires the Warden-Commander to explain it to him in the Wending Woods.
That said, Justice always had a fairly rigid view of justice in Amaranthine, so I don't think he really changed as much as Anders thinks. I do think being in a symbiotic relationship with Anders is quite different than possessing Kristoff's corpse, so it does lead to some very awkward situations, like with Ella.
I think Anders might benefit from speaking with the seers of Rivain, who have been said to commune (and sometimes merge) with spirits in a tradition that dates back to over a millennia.