If a diplomat is going on a shooting spree they can be shot and killed with no ensuing issues as a result of US law enforcement enforcing laws in the moment. If they are robbing a seven-eleven they can be arrested and held until arraignment. They can be ordered to remain in their home, aka house arrest. They can be ordered to leave and they have no option to remain in the country. The US constitution does not grant diplomats immunity it is something we have agreed to via treaty and law. They are 100% subject to our laws, that's why our law granting them immunity applies.