There is a fundamental misunderstanding regarding the role private security teams aboard ship. This article, Private Security Counters Pirates, is a good illustrations of these misunderstanding.
The role of private security teams is not to provide security, but to increase the level of security by supplementing the security all ready in place.
A ship at sea already has in place a chain of command, clearly defined responsibilities and duties, a trained crew, a watch schedule and a emergency response plan. The embarked security team must fit within this structure, not act independently from it.
The master of any ship which embarks a security team must ensure that an embarked security team team fully understands exactly what their role is aboard ship. Any action taken by the security team must be in accordance with the ship's security plan and be approved by the master. This is especially true in the case of a decision to use lethal force. Actions taken by the security team are just as much the responsibility of the master as actions taken by the ship's regular crew.