Asmodai
Oh shit......
ASMODEUS POISONTEETH Asmodai is the Master Interrogator-Chaplain of the Dark Angels. He is the oldest and most experienced of the Interrogator-Chaplains (hence his rank), and also the most obsessed with hunting down the Fallen Angels and making them repent. SHOWING HIS INTENSE BURNING LOYALTY TO THE GOD EMPEROR.
His obsession has grown and bled into the other aspects of his life as a Space Marine, and he's basically become the "fun police" of the 41st millennium. When a bunch of recruits didn't answer his questions as enthusiastically as he liked, he had them all killed because he thought their gene-seed might be contaminated. When he heard someone from the Seventh Company laughing while inside the Rock, he had the entire Company sworn to silence except for hymns and communicating in battle. He doesn't actually have the authority to do the second of those things, though. At least, not without Azrael's approval (which he won't get). Besides, a vow of silence covers speech. Laughing is not speaking. His preferred tool of interrogation is a torture weapon called "the Blades of Reason".
Azrael has had to rein him in or cover up for some of his more excessive actions, but he keeps Asmodai around because, say what you will about his sense of humor (or lack of it), he delivers -- if there's a Fallen Angel (or even someone who might know something about a Fallen Angel) out there, he will track it down.
Interesting despite his long serves and obsession he has only able to make two fallen repeat (that Interrogator-Chaplain form Dark Vengeance has more), implying he's really just a sadist asshole.
Quite ironically, Asmodai is named after a daemon king of lust in Christian mythology. So while WH40K Asmodai is busy fun-policing the galaxy, daemon Asmodai is conversely busy living it up, Slaaneshi-style.