Let the court decide. Jury of peers would see it as a threat. Get enough cases like this and cops would start to see it as normal fighting (If you claim assault, so will they) and both parties will start to get off with warnings. One of which will reconsider saying such things again.
If a tweet (or similar) makes a statement of fact, either mark it as "satire" in the body or provide the original source link backing up the statement (see rule 6)
Let the court decide. Jury of peers would see it as a threat. Get enough cases like this and cops would start to see it as normal fighting (If you claim assault, so will they) and both parties will start to get off with warnings. One of which will reconsider saying such things again.