Be very specifically and direct with your wording and with your intentions.
Think of it like this: You, the magick user, are like a sniper. The intended victim is like the terrorist/murderer/etc.
It is important to have a clear shot without innocent people in the way.
Let it be known that I think gun violence is a terrible thing, this is only an analogy.. and I highly discourage the use of magick or dangerous weapons for harming others. I condone neither and wish it was never neccesary so I am by no means encouraging it.
But I also believe that if a person threatens your well-being, you give them fair warning. If they continue, then do what it takes to make them stop. That is your right. Usually I use binding and reversing spells for situations like this. That way any harm that comes to the person is only brought on by their own actions.