When is it ok to kill a cop?

Let us assume that you are, for the most part, law abiding.
You haven't done anything worse than forget to pay the parking meter in a timely fashion.

The police have murdered your friend.
(yes, murder, as in planned, premeditated...)

It is more than one cop, and at least a DA.
And now the police are sniffing you out to see if you know anything.
Of course, if this goes to trial, it will be your word against theirs.