He's free. And that is good. The sadness you feel - is an issue of your ego wanting to possess: things, people, pets, and it has nothing to do with what Niki was. Understand that, and your pain will ease: it is not about him, it is about you. Don't think I am heartless. I am not. After having dealt with dying people and really suffering, traumatized people in the past, I just don't like to tell phrases, in the face of death they all mean nothing, and instead I tell you something of real value to help. The only lasting cures to all of life's sufferings and miseries are time, and understanding yourself.
BTW, we lost three dogs when I still lived with my parents, so I know how it's feeling (the pack of hunting dogs my grandfather had not counted). We come. We stay a while, we go. So it is, and it is neither good nor bad. But in the end, nothing has happened, nothing has moved.
Don't look down - look up!