The Intentional Death of Niko Yates

The Death of Niko Yates is a murder mystery with multiple angles. you may hear something from someone in one chapter and the same person say something in a different way from someone else's perspective.

Niko is a rich man.
William is a hard-working man.
Issac is a dedicated man.
Jessica is a fun-seeking woman.
John is a troubled man.
James is a noble butler.

Who could be the murderer of Niko?