The problem, here, is not the word whataboutism, nor is that word a showstopper for rational conversation. The point is not to shame the target in any way. The point is to bring the conversation back to the original point: Instead of "but what about..", talk to the actual point.
Which, so far, you've absolutely refused to do. At no point have you addressed the original point you tried to make, that there is some need to compare police brutality to something else.
So, let me walk back the comment on 'whataboutism' and instead of using the shorthand ask you why should we give any consideration to the question "what about other criminals' while discussing police brutality?
It appears that you're unwilling to stand honestly behind your own statements, nor willing to engage with the substance of the statements others have made, but rather would prefer to be evasive.
I would absolutely love to be proven wrong, here, but sadly fully expect not to be.