I think Bolvar will turn Evil someday and the Helm of domination will turn him over time.
The whole reason for this to me is that Arthas reverted back to himself the moment he lost the Helm and the blue light in his eyes faded.
My view is that if Arthas was truly the source of the Lich King's Evil he would never have reverted back to himself before dying.
I think Ner'Zhul or some other corrupting essence could still exist in the helm. If Frostmourne could be so all consuming of souls, what could the helm be?
Also To the people who say that Arthas killed Ner'Zhul consider the following:
Arthas not only destroyed Ner'Zhul in the book but also killed the little boy who represented himself and his humanity in the same book. Even after this we saw both the little boy in questing and Arthas reverted back to his humanity before death. So from that I would say Ner'Zhul's death is far from certain.
Also, Arthas had good intentions he took up Frostmourne from a desire to protect his people. Bolvar took up the helm of domination from a desire to protect the world.
Bolvar may yet fall from grace and the helm could turn out to be just as bad if not worse than frostmourne