Police do this to build a story retroactively -- we shot a person, so ... to make it excusable we arrest their "accomplices" because officer so and so will make up some report about how they threatened his life with a gun. If they don't arrest her, then it looks like murder. Also if she was arrested, her side of the story immediately is less believable -- "clearly officer had a good reason to arrest her, we can't trust her words".

This happened in another case, where a person was shot in a store (Wal-Mart perhaps). Police then proceeded to heavily interrogate the partner and try to get them to admit the victim was "unstable", "drunk", "violent" and so on. Because it builds an excusable story so the officer is cleared of charges quicker (they don't have to actually resign and move one town over to another dept., but just get a 10 days paid vacation).