When you use scrum, should you have scrum master in your team?

scrum is a simple project management tool to use. no need to scrum master.

I disagree with your answer. There should be  a scrum master to run the scrum. It may depend on the project, if the project is small then one of the team members may act as scrum master so there is no need of a separate scrum master role..

Absolutely. 

Scrum Master is not only the servant leader for the team, but also influence the wider organisation to adopt scrum principles. This is key role in the bigger organisations where scrum is not yet fully adopted.

Scrum Master should make sure that transparency, inspection and adoption are practiced by the team. Scrum Master should be an influential person who can sell the benefits of scrum pillars to the team the stakeholders and the wider organisation.

Scrum Master is one of the 3 roles within the scrum framework. 

Scrum Method defines 3 rôles : the first one an the most important for new Scrum teams is the Scrum Master, because he is the one who canalize the team and lead them to the best practices. A Scrum Team without SM is like a Soccer game without referee.