It's a bit like when fans of a club do something bad and other fans say "those idiots aren't real fans". No, they are, that's the problem. You have to accept that they actually are real fans of the same team you are, accept that they make the rest of you look bad and accept some responsibility in doing something about it by calling out that behaviour (or lesser forms of it) when you see it among yourselves. Just saying "they're not really one of us" when they do something bad makes it sound like you're washing your hands of responsibility and distancing yourself from the problem.
In this case the better line to take would be "Yes, he was a police officer and he abused that position. It's our responsibility to do better to make sure there's no place in the force for people like Couzens or the attitudes that allow people like Couzens to pass unnoticed." How much they can actually do is another matter, but the tone should be "we will learn from this".