Whatever the justification, impunity is unacceptable. It is a key part of the rule of law that everyone is subject to the law. To grant individuals impunity for whatever reason is unacceptable.  Even if diplomatic immunity is accepted, diplomats are less likely in their role to commit international crimes that a head of state or government is because diplomats do not have access to the coercive machinery that enable these crimes. Diplomatic relations can carry on very well without senior government figures being able to travel wherever they want to. Summits are a luxury not a necessity and most of the deals they make have been hammered out by the diplomats beforehand.