I hate to say this, but if the cop was off duty, is it really right to sue the department? Sure, get his ass fired and put in jail (don't inmates have a lovely time with cops among them?). But are they responsible for every moment of his day?

I know that sounds callous, but it's not supposed to.
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"Nature has constituted utility to man the standard and test of virtue. Men living in different countries, under different circumstances, different habits and regimens, may have different utilities; the same act, therefore, may be useful and consequently virtuous in one country which is injurious and vicious in another differently circumstanced" - Thomas Jefferson, moral relativist