"The end justifies the means" is a concept that has been very much distorted. What it originally meant was that, if you had a good aim in view, an action which was not in itself evil, would become a morally good action, whereas if you had a bad end in view, that same action would be morally bad.

It was nothing to do with the idea that an intrinsically evil action could become a good action just because it was done to achieve a good end, though that is the way it has been distorted over the years.

Rarelamb confirms what I wrote, expressing a position on this that is consistent. It is that the use of terror is justified, if it is a way of achieving a good end. It is the position which in practice dominated the 20th Century, and made it one of the bloodiest in history. As for the 21st Century, not much sign of any change there...