Hmm, was that after he said "for all they who take the sword shall perish with the sword?"
Barnes notes 3 possibilities...
1. They who resist by the sword the civil magistrate shall be punished; and it is dangerous, therefore, to oppose those who come with the authority of the civil ruler.
2. These men, Jews and Romans, who have taken the sword against the innocent, shall perish by the sword. God will take vengeance on them.
3. However, the most satisfactory interpretation is that which regards it as a caution to Peter. Peter was rash. Alone he had attacked the whole band. Jesus told him that his unseasonable and imprudent defense might be the occasion of his own destruction. In doing it he would endanger his life, for they who took the sword perished by it. This was probably a proverb, denoting that they who engaged in wars commonly perished there.