Who here played world of warcraft do you remember what Arthus did? His quotes? He would sacrifice everything to protect his homeland now think of this but replace Arthus with a librarian in his dire moment seeing all his battle brothers getting killed by the enemy Dire times call for Dire actions even if his soul is the price to pay to protect his battle brothers.
Also the end of the game Space Marine if you compleated it some space marines follow the codex as a set of guidlines to shape them but its what they do with the guidlines is what truely defines a space marine.
There you go a fluffy means to using daemonology as a Space marine