I just don't want Unalaq to be another "Chief of NWT isn't enough, I must dominate the world" villain. I have a feeling Vaatu has been feeding Unalaq lies and convinced him (years ago) to get rid of Tonraq to serve a purpose. Maybe told him that he was the only one who could unite Spirit and men again and if this wasn't done, something bad would happen (Seems possible?) Or even threaten him (idk)
In fact, Unalaq was already spiritbending by that time. What else did he know about spirits back then?