Scrum is a risk reduction framework for building complex products.
Scrum mitigates the risks associated to the traditional project management approach by distributing the responsibilities of the Project Manager role among the three Scrum roles.
In addition, it is important to remember that the Product Owner and the Scrum Master are separated roles and should be played by different individuals.
In fact, assigning the two roles to the same person increases risk and reduces self-organization.
Keep in mind: to reduce risks and conflicts the Product Owner and the Scrum Master should be assigned to two different people.