If the Emperor passes the test, by whatever metric and standard Mortarion judges these things, I will be pushing for actually getting on board with the Crusade. Because maybe mankind is more than an abstraction and an ideal to him, or an excuse made in the name of his own greater glory, and perchance might still be looking for a better way. If he is merely a man, even if he is the sum of all the dark glories of the Old Night, then there is at least hope.
We owe him nothing, I don't think we will act out of loyalty to the man at any point, but if a bitter thing like the Crusade is to happen I then it is an ill thing best done well. Its rather like how the condemned prays the headsman's stroke is steady and his blade sharp. Very few Primarchs committed to the Crusade purely on its own merit so this would be interesting indeed.
This is a very different circumstance than Angron.