I do not necessarily believe that he (not it, he) has become benevolent but PW states certain aspects of his character and since he has written him, I must assume they are correct, unless PW is deliberately trying to mislead us. He still appears to be drawn to human emotions, can sense them, and he seems to be drawn to people who are in distress and requiring comfort. When he first came to Cole, he was a terrified child, shut up in a dark dungeon and "I held his hand in the darkness until it was over." The prime motivating force as a pure spirit was compassion, bringing comfort to one who needed it.
However, he has developed since then, He advanced from his state at the beginning of the book until the end. He is no longer a pure spirit and since it is clear neither we, nor the majority of inhabitant of Thedas, have any real idea of the nature of spirits, or how they can interact with the material world, it is difficult to know truly what he is. It would certainly be wise to tread warily where he is concerned because he is still finding his way and is very impressionable, but to prejudge him as being totally untrustworthy and trying to deceive would be unfair, unless you are going to take that view about every companion.
The thing is PW states that you do not have to take him as a companion. If you feel like you do about him Fortlowe, just tell him to get lost the first time you meet him and he will oblige. To be honest that does not strike me as a character that is out to deceive you by pretending to be benevolent because such a character would not just walk away.