Maybe I’m not following your math here (no clue if your numbers are accurate, but since you believe they are, I’ll start there)
400/1000 police deaths a year are of black people. So 40% of deaths are of Blacks.
Of arrests: there are 10 million, only 3 million of which are black. So arrests of blacks are only 30%.
So 30% of arrests are of black people, but 40% of deaths are of black people – so you *must* agree it is literally disproportionate. (You can make some separate argument that they deserved it, but the numbers you present here do nothing to support that assertion)