“Well, the country from which the offender has legitimate diplomatic immunity must waive that immunity for prosecution here to proceed.”
Sure, but I’m not worried about prosecuting him for crimes committed while he had immunity. We should just unilaterally revoke his immunity, before he does something we really would like to prosecute him for.
The State Department would tell us that unilateral waiver of immunity isn’t workable because then other countries would do the same thing to our people in those countries, even when they are the victims of trumped-up charges.