"don't harm; don't be harmed" -
One of the clear reservations I have about stating the point that way is that "harm" seems overbroad. If a burglar or murderer threatens me or my friends, I consider myself entirely justified in placing a high degree of "harm" on that person to stop them from their efforts, and I think Epicurus would fully agree with that. How would you account for those situations in use of the word "harm"? Is it not "harming" a murderer to put a bullet between his eyes before he accomplishes his goal?