Better to err and think one an enemy than mistakenly let an enemy get too close. That happened during the Heresy and it almost destroyed the Imperium. [Kind of did, but that's above Gadriel's pay grade. Lord Calgar says everything's great, then everything's great. ]

I will take your word for it. [Who's going to say to a high king that he was lying? Not Gadriel. ] It is terrible enough to send brothers into battle without standing beside them. I cannot imagine how any Primarch feels sending his own sons.

That is the theory. I do not know for certain. Death is the end of our duty. That is all I know. This is a matter for Chaplains and I do not want to speak wrongly.