The best way to make this argument is to say that diplomatic immunity unnecessarily allows diplomats to break the laws of the countries they live in.
In this view, diplomatic immunity is not all that necessary for the practice of diplomacy. You can argue that there are only a very few countries that would charge foreign diplomats with crimes that they are not truly guilty of. Only a few countries would use criminal charges to harass diplomats from other countries. The anger that would be directed at these countries from the international community would prevent them from abusing foreign diplomats.
If diplomatic immunity is not necessary for diplomacy, then it is a bad thing. It allows people to get away with crimes simply because of the fact that they are diplomats. This is inequitable. If diplomatic immunity is both inequitable and unnecessary, it is a bad thing and should be done away with.