Taken from the point of view of his own people, someone like Cyrus was Great because;About every man named "the great". That title usually goes together with war and conquest. I don't know what's "great" with killing, pillaging and stealing land.
1. He was impactful, which is one way of measuring 'greatness' moreso than 'righteousness'.
2. He improved their lot. Sure, perhaps at the expense of others, but still...