"We shall make no excuses for the terror" is a memorable turn of phase but not an actual principle of communism, as evidenced by the fact that the immediate context of the quote makes an excuse for the terror:

When our turn comes, we shall not make excuses for the terror. But the royal terrorists, the terrorists by the grace of God and the law, are in practice brutal, disdainful, and mean, in theory cowardly, secretive, and deceitful, and in both respects disreputable.

It's saying revolutionary violence directed at overturning oppressive conditions is justified by the violence of oppression (see also Mark Twain's "two reigns of terror" quote). It is not saying "we'll be as violent as we want against whoever we want with no need to justify our actions."