So the Dark Brotherhood's standards dropped that day...

[Megatarius]

Think of it this way:

 

If you sap morale, then the opponent is afraid to fight you. Killing them then would be murder because they aren't defending themselves, and neither are you.

[Axxaw]

The thing about all these exceptions is that the characters are Innately hostile towards you, so if you sneak kill them without them detecting you or demoralize them should it really count as murder in the eyes of the night mother even though they would definitely attack you first? In real life it would be yes of course but I think the logic for video games should be a little different, especially the arena fighters, that should never count since both combatants signed up for a death match and getting cold feet isn't going to get you out of it once you've started. As far as Jalbert, he is a necromancer and necromancers are considered one of the "evil" factions yes? I remember a let's player on youtube doing the Dark Brotherhood and he kill Hilod the Outlaw in the Waterfront district and it didn't register as murder (he was playing the x360 version). If you can get into the dark brotherhood by killing Jalbert then I'd consider it a bug to be fixed.

[Ferryt]

I actually agree with you, Axxaw. Jalbert is going to attack you the instant you meet him face-to-face, or at least after what little he has has to say to you. I don't think there are any real dialog options with him. I'm not sure why killing Hilod the Outlaw wouldn't register as murder, either. He's a member of the "Thieves Guild", "Imperial City Waterfront District Residents", and "IC Citizens" factions, if my memory serves me, so he's NOT a member of one of the "evil" factions, unless the Thieves Guild is considered as such, which I don't think it is. I'm not sure how to fix the bug with Jalbert, or even if it can be fixed, since this doesn't seem to be anything defined in the CS, but, rather, some internal workings of the game engine. As far as I'm concerned, Jalbert is one of the Bad Guys and deserves to be put down, so his "assassination" should not be considered murder. As for arena fighters, again, it's explicitly stated that the fights are to the death. Seeing how many people get killed over the course of time in the Arena, you'd think the Dark Brotherhood would be overflowing with operatives if those deaths were to be classified as murders. This, also, is bugged, but, again, I have no idea whether or not it can even be fixed, let alone find someone willing to try.

 

My only explanation is that the murder tag comes into effect with named NPCs. It seems to be the only unusual factor, here, since I killed the bandits outside of Vilverin as well as everyone inside Vilverin, most of them attacked before they detected me, and only Jalbert triggered the visit from Lucien. I was wondering about the one-shot kill of Jalbert, but I've killed lots of un-named bandits, marauders, conjurers, and necromancers in this game on one shot, undetected, and none of them ever counted as murder -- just the one with a name. Other than that, I'm not sure what to say. The wiki (I just checked it) on the Dark Brotherhood gives a list of recommended hits. Jalbert isn't on it.