I think that kinda depends on how you view morals and ethics. One could claim that a profession doesn't define your morality.......but I doubt many say that about the Nazi or Isis.

Now that's a bit hyperbolic, but it does lend weight to the framework of the argument. Can an individual be "good" and simultaneously be supporting a system or group that is "bad".

Now you can wholly reject the notion that the police are bastards, but that would just be a different debate, and likely a futile one. But, it would still be a stronger argument than your original.

moonlights as both an EMT and a firefighter when she's not a cop.

If my day job is working as a corrupt politician, but I moonlight at a homeless shelter.....does that cancel each other out? If a person wanted for murder stops a mugging, does he get a get out of jail free card?

She's normally a really great person

I don't doubt she's a great person to you and people she cares about, but we're not really talking about personal relationships.