I think the Buffalo incident yesterday[0] gives us a good indication of just how rare a good cop is. Even though there were dozens of cops that witnessed it, they all lied and claimed that "During that skirmish involving protestors, one person was injured when he tripped & fell".

Then, after the cops who nearly killed the man got suspended, all 57 people on that particular team resigned "in disgust because of the treatment of two of their members, who were simply executing orders,"[2].

Since you want empirical results, that is 0 good cops out of the 57 people on the force. We can employ the rule of 3 to establish an upper bound good cop rate as 5.2% (95% CI).

This isn't to say that the cops are by nature bad. Rather, the institutions and norms that exist in many police departments ensure that nearly all cops become bad cops.

[0]: https://news.wbfo.org/post/graphic-video-two-buffalo-police-...

[1]: https://www.investigativepost.org/2020/06/05/police-unit-res...

[2]: https://en.wikipedia.org/wiki/Rule_of_three_(statistics)