Simply stating that the Crusades invalidate the principles of Christianity is falling for this "Abusus non tollit usum" logical error.

Actually that is not what I'm asking. I'm asking to what extent we can judge Christianity because events were (a) carried out by Christians, (b) initiated by Christian leaders, and/or (c) justified by appeals to Christian teaching. My critique is also not meant to involve just Christianity, but all religions.

We can argue after the fact that the Crusades or the actions of the Conquistadores or slavetraders were not consistent with Christian reasoning, but they were to some extent initiated or blessed by Christian leaders at the time, were considered "good Christian things to do," and were justified with appeals to the Bible.

This would be weighed against, for example, the long history of altruism conducted by Catholics, Mennonites, Quakers, Protestant missionaries, etc. Surely these good things are used by many Christians as evidence of the goodness which comes from the Christian faith.

I'm particularly concerned with the possibility of using scripture to justify deeds both good and bad.