The crusaders were acting in disobedience to God. Did they know that what they were doing was wrong, but chose to do it anyway?

This text is from a transcript of a talk by David Gooding, entitled ‘Are You Worth More than a Pig?’ (2007).

Just to restate your question to make sure I've understood, you're asking: When they went to slaughter the Turks, did all the crusaders know they were doing wrong?

Well, this is not meant to be a 'get-out' or cynical, but I can't tell you what was in their minds when they did it! I think a lot of them had been persuaded by people that should have known better that, somehow or other, the use of the sword to promote Christianity was justifiable.

You got that kind of thing with Constantine, when he had the sign of the cross put on his military banners and said, 'In this sign, I conquer.'

Whether the knights and the footmen who went on the Crusades knew it was wrong or not, I don't know. Some of their leaders should have known, if they had read Scripture. Our Lord forbade his apostles to use the sword, either to protect or to promote his kingdom (Matthew 26:52). And so it remains today.